diff --git a/.env.example b/.env.example
index 710a9ec..6883c88 100644
--- a/.env.example
+++ b/.env.example
@@ -81,6 +81,18 @@ S3_BUCKET=ekklesia-development
 # EKKLESIA_SMTP_PASSWORD=
 # EKKLESIA_SMTP_FROM_ADDRESS=noreply@example.org
 # EKKLESIA_SMTP_FROM_NAME=Ekklesia
+#
+# EKKLESIA_SMTP_HELO_NAME is the name Ekklesia announces itself as in the SMTP EHLO command.
+#
+# Leave it unset unless a relay refuses the connection. An SMTP client has to identify itself, and
+# with nothing configured Nodemailer derives that from the machine hostname — substituting the
+# address literal [127.0.0.1] whenever the hostname is not fully qualified, which is the normal
+# state of a container or an unnamed VM. Some relays close the connection on that greeting with a
+# 421 before any message is offered, which looks exactly like the relay being unreachable.
+#
+# Set it to a fully qualified domain name your deployment owns — typically the sending domain.
+# No scheme, no port, no brackets, and at least one dot; anything else is refused at startup.
+# EKKLESIA_SMTP_HELO_NAME=example.org
 
 # Where an emailed verification or password-reset link sends a browser (ADR 0038).
 #
diff --git a/apps/api/tests/host-configuration-parity.test.ts b/apps/api/tests/host-configuration-parity.test.ts
index 1f662e4..56c0aa5 100644
--- a/apps/api/tests/host-configuration-parity.test.ts
+++ b/apps/api/tests/host-configuration-parity.test.ts
@@ -33,6 +33,7 @@ function environment(
 
 const configuredInfrastructure = Object.freeze({
   EKKLESIA_SMTP_FROM_ADDRESS: "noreply@example.org",
+  EKKLESIA_SMTP_HELO_NAME: "mail.example.org",
   EKKLESIA_SMTP_HOST: "smtp.example.org",
   EKKLESIA_SMTP_PASSWORD: smtpPassword,
   EKKLESIA_SMTP_USERNAME: "relay@example.org",
@@ -77,6 +78,9 @@ describe("host configuration parity between startApi and Doctor", () => {
 
     expect(apiView(env).mail).toEqual(doctorView(env).mail);
     expect(apiView(env).mail).toMatchObject({
+      // The SMTP client identity is part of that equivalence: a relay that refuses the greeting
+      // must fail in Doctor for the same reason it fails at runtime.
+      heloName: "mail.example.org",
       host: "smtp.example.org",
       port: 587,
       security: "starttls",
@@ -123,6 +127,10 @@ describe("host configuration parity between startApi and Doctor", () => {
         "MAIL_USERNAME_WITHOUT_PASSWORD",
       ],
       [{ ...configuredInfrastructure, EKKLESIA_SMTP_PORT: "70000" }, "MAIL_PORT_INVALID"],
+      [
+        { ...configuredInfrastructure, EKKLESIA_SMTP_HELO_NAME: "localhost" },
+        "MAIL_HELO_NAME_INVALID",
+      ],
     ] as const) {
       const env = environment(overrides);
       expect(() => apiView(env)).toThrowError(expect.objectContaining({ code }));
diff --git a/core/src/host/configuration.ts b/core/src/host/configuration.ts
index 5e7a8ae..1d8c78e 100644
--- a/core/src/host/configuration.ts
+++ b/core/src/host/configuration.ts
@@ -133,6 +133,10 @@ export function loadHostConfiguration(environment: HostEnvironment): HostConfigu
     mail: {
       fromAddress: environment.EKKLESIA_SMTP_FROM_ADDRESS,
       fromName: environment.EKKLESIA_SMTP_FROM_NAME,
+      // The name this client announces in `EHLO`. Optional, and unset is the ordinary case: it
+      // matters only where the machine's own hostname is not a fully qualified name the relay will
+      // accept.
+      heloName: environment.EKKLESIA_SMTP_HELO_NAME,
       host: environment.EKKLESIA_SMTP_HOST,
       password: environment.EKKLESIA_SMTP_PASSWORD,
       port: environment.EKKLESIA_SMTP_PORT,
diff --git a/core/src/mail/config.ts b/core/src/mail/config.ts
index 3262866..cefdc0f 100644
--- a/core/src/mail/config.ts
+++ b/core/src/mail/config.ts
@@ -8,6 +8,7 @@ export type MailConfigErrorCode =
   | "MAIL_FROM_ADDRESS_INVALID"
   | "MAIL_FROM_ADDRESS_MISSING"
   | "MAIL_FROM_NAME_INVALID"
+  | "MAIL_HELO_NAME_INVALID"
   | "MAIL_HOST_INVALID"
   | "MAIL_PASSWORD_WITHOUT_USERNAME"
   | "MAIL_PORT_INVALID"
@@ -19,6 +20,8 @@ const mailConfigMessages: Readonly<Record<MailConfigErrorCode, string>> = Object
   MAIL_FROM_ADDRESS_INVALID: "The outbound email sender address is not a valid email address.",
   MAIL_FROM_ADDRESS_MISSING: "A sender address is required when an SMTP host is set.",
   MAIL_FROM_NAME_INVALID: `The sender display name must be at most ${MAIL_MAX_DISPLAY_NAME_LENGTH} characters and contain no quotes, backslashes, or control characters.`,
+  MAIL_HELO_NAME_INVALID:
+    "The SMTP HELO name must be a fully qualified domain name, with no scheme, port, brackets, or spaces.",
   MAIL_HOST_INVALID: "The SMTP host must be a bare hostname or IP address, with no scheme or port.",
   MAIL_PASSWORD_WITHOUT_USERNAME:
     "An SMTP password requires a username; supply both or neither for an unauthenticated relay.",
@@ -70,6 +73,7 @@ export type MailSecurityMode = (typeof MAIL_SECURITY_MODES)[number];
 
 export interface MailProviderConfigInput {
   readonly host?: string | undefined;
+  readonly heloName?: string | undefined;
   readonly port?: string | number | undefined;
   readonly security?: string | undefined;
   readonly username?: string | undefined;
@@ -87,6 +91,24 @@ export interface MailProviderConfigInput {
  */
 export interface MailProviderConfig {
   readonly host: string;
+  /**
+   * The name this client identifies itself as in the SMTP `EHLO` command, when the deployment sets
+   * one.
+   *
+   * This exists because of a real failure, and the failure is generic rather than tied to any one
+   * provider. An SMTP client must greet the server with its own identity, and when nothing is
+   * configured Nodemailer derives it from the machine's hostname — falling back to the address
+   * literal `[127.0.0.1]` whenever that hostname is not fully qualified, which is the normal state
+   * of a container or a VM nobody has named. Some relays refuse that greeting outright, closing the
+   * connection with a `421` before any message is offered, which surfaces here as a bare
+   * `MAIL_PROVIDER_UNAVAILABLE` and looks indistinguishable from the relay being down.
+   *
+   * Absent means Nodemailer's own derivation is used, unchanged. It is deliberately not defaulted to
+   * the sender's domain: a host whose machine hostname *is* a valid FQDN may already be greeting
+   * correctly — some relays check it against the connecting address's reverse DNS — and silently
+   * replacing a working identity would trade one deployment's bug for another's.
+   */
+  readonly heloName?: string;
   readonly port: number;
   readonly security: MailSecurityMode;
   /** Present only for an authenticated relay. A relay authorized by source IP — the common Google
@@ -119,6 +141,29 @@ function validateHost(host: string): string {
   return trimmed;
 }
 
+/**
+ * A fully qualified domain name, and deliberately nothing else.
+ *
+ * Stricter than `HOST_PATTERN` on purpose, in two ways. It requires **at least one dot**, because a
+ * single unqualified label is precisely the input Nodemailer already refuses to use — configuring
+ * one would reproduce the failure this setting exists to fix. And it requires the final label to
+ * begin with a letter, which rejects a bare IPv4 address: `EHLO 192.0.2.1` is not valid SMTP syntax,
+ * and the bracketed address-literal form that *is* valid is the greeting the affected relays reject.
+ *
+ * Everything the requirement rules out falls out of the anchors and the character class rather than
+ * from a list of special cases: a scheme, a port, a path, whitespace, brackets, a comma, a
+ * carriage return or line feed, and any other prose all fail to match.
+ */
+const HELO_NAME_PATTERN =
+  /^(?=.{1,253}$)(?:[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?\.)+[a-z](?:[a-z0-9-]{0,61}[a-z0-9])?$/iu;
+
+function validateHeloName(heloName: string): string {
+  if (!HELO_NAME_PATTERN.test(heloName)) {
+    throw new MailConfigError("MAIL_HELO_NAME_INVALID");
+  }
+  return heloName;
+}
+
 function validatePort(port: string | number | undefined, security: MailSecurityMode): number {
   if (port === undefined || (typeof port === "string" && port.trim() === "")) {
     return DEFAULT_PORTS[security];
@@ -174,6 +219,16 @@ export function createMailProviderConfig(
   const validatedHost = validateHost(host);
   const port = validatePort(input?.port, security);
 
+  // Validated exactly as supplied, with no trimming. Every other value here tolerates surrounding
+  // whitespace as a copy-paste artefact; this one must not, because a stray character in the value
+  // goes straight onto the wire in a protocol command. It is refused rather than repaired, the same
+  // rule an address follows.
+  const heloNameInput = input?.heloName;
+  const heloName =
+    heloNameInput === undefined || heloNameInput === ""
+      ? undefined
+      : validateHeloName(heloNameInput);
+
   const username = input?.username?.trim() === "" ? undefined : input?.username?.trim();
   const password = input?.password === "" ? undefined : input?.password;
   if (username !== undefined && password === undefined) {
@@ -201,6 +256,7 @@ export function createMailProviderConfig(
     host: validatedHost,
     port,
     security,
+    ...(heloName === undefined ? {} : { heloName }),
     ...(fromName === undefined ? {} : { fromName }),
     ...(username === undefined || password === undefined ? {} : { password, username }),
   });
@@ -219,7 +275,11 @@ export function describeMailProvider(config: MailProviderConfig): string {
     config.fromName === undefined
       ? config.fromAddress
       : `${config.fromName} <${config.fromAddress}>`;
+  // The HELO name is host infrastructure, not a credential: it is the identity this client announces
+  // in the clear on every connection, so printing it costs nothing and is exactly what an operator
+  // needs when a relay has refused the greeting.
+  const helo = config.heloName === undefined ? "" : ` · HELO ${config.heloName}`;
   return `SMTP ${config.host}:${config.port} · ${config.security} · from ${sender} · ${
     config.username === undefined ? "unauthenticated relay" : "authenticated"
-  }`;
+  }${helo}`;
 }
diff --git a/core/src/mail/smtp-provider.ts b/core/src/mail/smtp-provider.ts
index 323f966..a72cf05 100644
--- a/core/src/mail/smtp-provider.ts
+++ b/core/src/mail/smtp-provider.ts
@@ -126,6 +126,11 @@ function transportOptions(
     disableUrlAccess: true,
     greetingTimeout: bounds.greetingTimeoutMs,
     host: config.host,
+    // The identity this client announces in `EHLO`. Left to Nodemailer when the deployment
+    // configures none — it derives one from the machine hostname, falling back to the address
+    // literal `[127.0.0.1]` when that hostname is not fully qualified, which some relays refuse
+    // with a `421` before a message is ever offered (ADR 0037).
+    ...(config.heloName === undefined ? {} : { name: config.heloName }),
     // Deliberately not a pooled transport: `pool` is absent, which is Nodemailer's default and is
     // what this type permits. A pool keeps sockets open and reuses them across sends, which for a
     // platform sending a handful of transactional messages buys nothing and makes the failure modes
diff --git a/core/tests/doctor-host-configuration.test.ts b/core/tests/doctor-host-configuration.test.ts
index 032a7ee..dfa01a4 100644
--- a/core/tests/doctor-host-configuration.test.ts
+++ b/core/tests/doctor-host-configuration.test.ts
@@ -363,6 +363,36 @@ describe("Doctor host-configuration parity — outbound email regression", () =>
     expect(JSON.stringify(mailCheck)).not.toContain(smtpPassword);
   });
 
+  it("probes with the SMTP client identity the runtime would use, and reports it", async () => {
+    const services = dependencies();
+
+    const { report } = await diagnose(
+      environment({ ...mailEnvironment, EKKLESIA_SMTP_HELO_NAME: "mail.example.org" }),
+      services,
+    );
+    const mailCheck = check(report, "outbound-email.configuration");
+
+    // Doctor builds its provider from the same validated configuration `startApi` does, so a relay
+    // that refuses the greeting fails here rather than only when a reset email silently never
+    // arrives. The name is announced in the clear on every connection, so showing it leaks nothing.
+    expect(services.probeMail).toHaveBeenCalledWith(
+      expect.objectContaining({ heloName: "mail.example.org" }),
+    );
+    expect(mailCheck.details[0]).toContain("HELO mail.example.org");
+    expect(JSON.stringify(mailCheck)).not.toContain(smtpPassword);
+  });
+
+  it("reports a malformed SMTP client identity as a typed failure, naming no value", async () => {
+    const { report } = await diagnose(
+      environment({ ...mailEnvironment, EKKLESIA_SMTP_HELO_NAME: "smtp://internal-host:2525" }),
+    );
+    const mailCheck = check(report, "outbound-email.configuration");
+
+    expect(mailCheck.status).toBe("failure");
+    expect(mailCheck.details[0]).toBe("The SMTP settings were rejected: MAIL_HELO_NAME_INVALID.");
+    expect(JSON.stringify(mailCheck)).not.toContain("internal-host");
+  });
+
   it("leaves a malformed SMTP setting reported as a typed configuration failure", async () => {
     const { report } = await diagnose(
       environment({ ...mailEnvironment, EKKLESIA_SMTP_PORT: "70000" }),
diff --git a/core/tests/host-configuration-boundaries.test.ts b/core/tests/host-configuration-boundaries.test.ts
index 20c5326..63f9399 100644
--- a/core/tests/host-configuration-boundaries.test.ts
+++ b/core/tests/host-configuration-boundaries.test.ts
@@ -71,6 +71,7 @@ const INTERPRETED_VARIABLES = Object.freeze([
   "EKKLESIA_PUBLIC_SIGNUP_ENABLED",
   "EKKLESIA_SMTP_FROM_ADDRESS",
   "EKKLESIA_SMTP_FROM_NAME",
+  "EKKLESIA_SMTP_HELO_NAME",
   "EKKLESIA_SMTP_HOST",
   "EKKLESIA_SMTP_PASSWORD",
   "EKKLESIA_SMTP_PORT",
diff --git a/core/tests/host-configuration.test.ts b/core/tests/host-configuration.test.ts
index 0e9eabf..39dec57 100644
--- a/core/tests/host-configuration.test.ts
+++ b/core/tests/host-configuration.test.ts
@@ -20,6 +20,7 @@ const productionEnvironment = Object.freeze({
   EKKLESIA_MODULE_SECRET__ANNOUNCEMENTS__STORAGE_CREDENTIAL: "module-secret-material",
   EKKLESIA_SMTP_FROM_ADDRESS: "noreply@example.org",
   EKKLESIA_SMTP_FROM_NAME: "Ekklesia",
+  EKKLESIA_SMTP_HELO_NAME: "mail.example.org",
   EKKLESIA_SMTP_HOST: "smtp.example.org",
   EKKLESIA_SMTP_PASSWORD: "sup3r-secret-relay-password",
   EKKLESIA_SMTP_PORT: "587",
@@ -67,6 +68,7 @@ describe("host configuration loading", () => {
     expect(host.mail).toEqual({
       fromAddress: "noreply@example.org",
       fromName: "Ekklesia",
+      heloName: "mail.example.org",
       host: "smtp.example.org",
       password: "sup3r-secret-relay-password",
       port: "587",
@@ -80,6 +82,9 @@ describe("host configuration loading", () => {
     });
     expect(relayByAddress.mail.username).toBeUndefined();
     expect(relayByAddress.mail.password).toBeUndefined();
+    // The SMTP client identity is optional too, and unset is the ordinary case: it matters only
+    // where the machine's own hostname is not a fully qualified name the relay accepts.
+    expect(relayByAddress.mail.heloName).toBeUndefined();
     // A source-IP-authorized relay has no credentials at all, and that must still validate.
     expect(createMailProviderConfig(relayByAddress.mail)).toBeDefined();
   });
diff --git a/core/tests/mail-boundaries.test.ts b/core/tests/mail-boundaries.test.ts
index 85b1829..67a34c2 100644
--- a/core/tests/mail-boundaries.test.ts
+++ b/core/tests/mail-boundaries.test.ts
@@ -161,15 +161,44 @@ describe("outbound email stays server-only", () => {
   });
 
   it("hardcodes no provider hostname in Core", async () => {
-    // Google Workspace is a deployment target, not the architecture.
+    // Google Workspace is a deployment target, not the architecture. It is the deployment that
+    // exposed the generic `EHLO` defect the HELO-name setting fixes, and it is named nowhere the
+    // code could come to depend on it.
     for (const file of await readTextFiles("core/src/mail")) {
-      expect(file.text.toLowerCase(), file.path).not.toContain("gmail");
-      expect(file.text.toLowerCase(), file.path).not.toContain("smtp-relay");
-      expect(file.text.toLowerCase(), file.path).not.toContain("resend");
-      expect(file.text.toLowerCase(), file.path).not.toContain("brevo");
+      const text = file.text.toLowerCase();
+      for (const forbidden of [
+        "gmail",
+        "smtp-relay",
+        "googlemail",
+        "google.com",
+        "outlook",
+        "office365",
+        "sendgrid",
+        "mailgun",
+        "postmark",
+        "resend",
+        "brevo",
+      ]) {
+        expect(text, `${file.path} names ${forbidden}`).not.toContain(forbidden);
+      }
     }
   });
 
+  it("defaults the SMTP client identity to nothing, so no deployment's name is baked in", async () => {
+    const config = await readFile(path.join(repositoryRoot, "core/src/mail/config.ts"), "utf8");
+    const provider = await readFile(
+      path.join(repositoryRoot, "core/src/mail/smtp-provider.ts"),
+      "utf8",
+    );
+
+    // The value that fixed a real deployment is configuration, never a constant. A fallback literal
+    // here would be one church's domain compiled into every installation.
+    expect(config).not.toContain("riverofglorymn");
+    expect(provider).not.toContain("riverofglorymn");
+    // The provider passes it through and supplies nothing of its own when it is absent.
+    expect(provider).toContain("config.heloName === undefined ? {} : { name: config.heloName }");
+  });
+
   it("never disables TLS certificate verification", async () => {
     for (const file of await readTextFiles("core/src/mail")) {
       expect(file.text, file.path).not.toContain("rejectUnauthorized");
diff --git a/core/tests/mail-contract.test.ts b/core/tests/mail-contract.test.ts
index cb0d7dc..9aca67d 100644
--- a/core/tests/mail-contract.test.ts
+++ b/core/tests/mail-contract.test.ts
@@ -193,6 +193,116 @@ describe("host mail configuration", () => {
   });
 });
 
+describe("SMTP client identity (HELO name)", () => {
+  /**
+   * A real deployment defect, and a generic one.
+   *
+   * An SMTP client must greet the server with its own identity. Nodemailer derives that from the
+   * machine hostname and falls back to the address literal `[127.0.0.1]` when the hostname is not
+   * fully qualified — the normal state of a container. Some relays close the connection on that
+   * greeting with a `421` before any message is offered, which reaches a caller as an
+   * indistinguishable `MAIL_PROVIDER_UNAVAILABLE`. The deployment can now say what to greet as.
+   */
+  it("omits the setting entirely when the deployment configures none", () => {
+    const config = createMailProviderConfig(validConfig);
+
+    // Absent means Nodemailer's own derivation, unchanged. The key is not present at all rather
+    // than present-and-undefined, so nothing downstream can mistake it for a configured empty name.
+    expect(config).not.toHaveProperty("heloName");
+    expect(createMailProviderConfig({ ...validConfig, heloName: "" })).not.toHaveProperty(
+      "heloName",
+    );
+    expect(createMailProviderConfig({ ...validConfig, heloName: undefined })).not.toHaveProperty(
+      "heloName",
+    );
+  });
+
+  it.each([
+    ["a two-label domain", "example.org"],
+    ["a subdomain", "mail.example.org"],
+    ["a deep subdomain", "smtp.relay.mail.example.org"],
+    ["hyphens inside labels", "out-bound.example-church.org"],
+    ["digits", "mx1.example2.org"],
+    ["mixed case", "Mail.Example.ORG"],
+  ])("accepts %s", (_label, heloName) => {
+    expect(createMailProviderConfig({ ...validConfig, heloName })).toMatchObject({ heloName });
+  });
+
+  it.each([
+    ["a scheme", "smtp://example.org"],
+    ["a port", "example.org:587"],
+    ["a path", "example.org/relay"],
+    ["a space", "example org"],
+    ["a leading space", " example.org"],
+    ["a trailing space", "example.org "],
+    ["a carriage return", "example.org\r"],
+    ["a line feed", "example.org\nEHLO evil.example.net"],
+    ["a tab", "example.org\t"],
+    ["a comma", "example.org,evil.example.net"],
+    ["an unqualified single label", "localhost"],
+    ["a machine name with no domain", "ekklesia-host"],
+    ["a bracketed address literal", "[127.0.0.1]"],
+    ["a bare IPv4 address", "192.0.2.1"],
+    ["a trailing dot", "example.org."],
+    ["a leading dot", ".example.org"],
+    ["consecutive dots", "example..org"],
+    ["a leading hyphen in a label", "-example.org"],
+    ["a trailing hyphen in a label", "example-.org"],
+    ["an email address", "noreply@example.org"],
+    ["prose", "our church mail server"],
+    ["nothing but dots", "..."],
+    ["an over-long name", `${"a".repeat(250)}.example.org`],
+  ])("refuses %s", (_label, heloName) => {
+    expect(() => createMailProviderConfig({ ...validConfig, heloName })).toThrow(
+      expect.objectContaining({ code: "MAIL_HELO_NAME_INVALID" }),
+    );
+  });
+
+  it("refuses an unqualified name, because that is the input the fallback already rejects", () => {
+    // Configuring a single label would reproduce the very failure this setting exists to fix:
+    // Nodemailer discards a non-FQDN hostname and substitutes the address literal.
+    expect(() => createMailProviderConfig({ ...validConfig, heloName: "ekklesia" })).toThrow(
+      MailConfigError,
+    );
+  });
+
+  it("never carries the offending value in the error", () => {
+    try {
+      createMailProviderConfig({ ...validConfig, heloName: "smtp://secret-internal-name:2525" });
+      throw new Error("expected a refusal");
+    } catch (error) {
+      const serialized = `${(error as Error).message} ${(error as Error).stack ?? ""}`;
+      expect(serialized).not.toContain("secret-internal-name");
+      expect(serialized).not.toContain("2525");
+    }
+  });
+
+  it("is described to an operator, because it is infrastructure rather than a credential", () => {
+    const described = describeMailProvider(
+      createMailProviderConfig({
+        ...validConfig,
+        heloName: "mail.example.org",
+        password,
+        username: "relay@example.org",
+      }) as Parameters<typeof describeMailProvider>[0],
+    );
+
+    // It is announced in the clear on every connection, so printing it leaks nothing — and it is
+    // exactly what an operator needs when a relay has refused the greeting.
+    expect(described).toContain("HELO mail.example.org");
+    expect(described).not.toContain(password);
+    expect(described).not.toContain("relay@example.org");
+  });
+
+  it("says nothing about a HELO name when none is configured", () => {
+    const described = describeMailProvider(
+      createMailProviderConfig(validConfig) as Parameters<typeof describeMailProvider>[0],
+    );
+
+    expect(described).not.toContain("HELO");
+  });
+});
+
 describe("transactional message validation", () => {
   const message = {
     subject: "Verify your email address",
diff --git a/core/tests/mail-smtp.integration.test.ts b/core/tests/mail-smtp.integration.test.ts
index 2b56112..9613f71 100644
--- a/core/tests/mail-smtp.integration.test.ts
+++ b/core/tests/mail-smtp.integration.test.ts
@@ -1,4 +1,4 @@
-import type { AddressInfo } from "node:net";
+import type { AddressInfo, Socket } from "node:net";
 import { createServer, type Server } from "node:net";
 import { SMTPServer } from "smtp-server";
 import { afterEach, describe, expect, it } from "vitest";
@@ -25,6 +25,8 @@ interface CapturedMessage {
   readonly from: string;
   readonly to: readonly string[];
   readonly data: string;
+  /** The argument this client actually sent in `EHLO`, lowercased by the server. */
+  readonly greetedAs: string;
 }
 
 interface Harness {
@@ -52,6 +54,14 @@ interface HarnessOptions {
   readonly rejectRecipients?: readonly string[];
   /** Advertises STARTTLS. Off by default, so a `starttls` client must refuse to continue. */
   readonly offerStartTls?: boolean;
+  /**
+   * Refuses any client whose `EHLO` argument is not exactly this name, as a relay that requires a
+   * fully qualified client identity does.
+   *
+   * The refusal is a `421`, the same class of response the real relay returned: a temporary failure
+   * that closes the connection before a message is offered.
+   */
+  readonly requireGreeting?: string;
 }
 
 async function startSmtpServer(options: HarnessOptions = {}): Promise<Harness> {
@@ -81,11 +91,26 @@ async function startSmtpServer(options: HarnessOptions = {}): Promise<Harness> {
         captured.push({
           data,
           from: session.envelope.mailFrom === false ? "" : session.envelope.mailFrom.address,
+          greetedAs: session.hostNameAppearsAs,
           to: session.envelope.rcptTo.map((entry) => entry.address),
         });
         callback();
       });
     },
+    onMailFrom(_address, session, callback) {
+      if (
+        options.requireGreeting !== undefined &&
+        session.hostNameAppearsAs !== options.requireGreeting.toLowerCase()
+      ) {
+        const error = new Error("Try again later, closing connection.") as Error & {
+          responseCode?: number;
+        };
+        error.responseCode = 421;
+        callback(error);
+        return;
+      }
+      callback();
+    },
     onRcptTo(address, _session, callback) {
       if (options.rejectRecipients?.includes(address.address)) {
         // 550 is a permanent refusal of this mailbox, which is what a real relay returns.
@@ -107,6 +132,64 @@ async function startSmtpServer(options: HarnessOptions = {}): Promise<Harness> {
   return { captured, close, port: address.port };
 }
 
+/**
+ * A relay that refuses the greeting itself, byte for byte as the real one did.
+ *
+ * `smtp-server` exposes no `EHLO` hook — its earliest usable one is `MAIL FROM`, which
+ * `transporter.verify()` never reaches — so modelling this failure at the point it actually happens
+ * needs a socket that speaks the handful of SMTP verbs involved. That matters: the whole reason the
+ * defect was hard to diagnose is that it aborts the session *before* a message is offered, which is
+ * also why Doctor's send-nothing probe can see it.
+ */
+async function startGreetingStrictRelay(expected: string): Promise<Harness> {
+  const sockets = new Set<Socket>();
+  const server = createServer((socket) => {
+    sockets.add(socket);
+    socket.on("close", () => sockets.delete(socket));
+    socket.on("error", () => undefined);
+    socket.write("220 relay.invalid ESMTP\r\n");
+    let buffer = "";
+    socket.on("data", (chunk: Buffer) => {
+      buffer += chunk.toString("utf8");
+      let index = buffer.indexOf("\r\n");
+      while (index !== -1) {
+        const line = buffer.slice(0, index);
+        buffer = buffer.slice(index + 2);
+        const [verb = "", argument = ""] = line.split(" ");
+        if (verb.toUpperCase() === "EHLO" || verb.toUpperCase() === "HELO") {
+          if (argument.toLowerCase() !== expected.toLowerCase()) {
+            // The exact shape the deployment saw: a 421 naming no reason, then the door shut.
+            socket.write("421-4.7.0 Try again later, closing connection. (EHLO)\r\n");
+            socket.write("421 4.7.0 Goodbye\r\n");
+            socket.end();
+            return;
+          }
+          socket.write("250-relay.invalid\r\n250 OK\r\n");
+        } else if (verb.toUpperCase() === "QUIT") {
+          socket.write("221 Bye\r\n");
+          socket.end();
+          return;
+        } else {
+          socket.write("250 OK\r\n");
+        }
+        index = buffer.indexOf("\r\n");
+      }
+    });
+  });
+
+  await new Promise<void>((resolve) => server.listen(0, "127.0.0.1", resolve));
+  const address = server.address() as AddressInfo;
+  const close = async (): Promise<void> => {
+    // `close()` waits for open sockets, and a refused client may still be holding one.
+    for (const socket of sockets) {
+      socket.destroy();
+    }
+    await new Promise<void>((resolve) => server.close(() => resolve()));
+  };
+  openHarnesses.push(close);
+  return { captured: [], close, port: address.port };
+}
+
 function config(port: number, overrides: Record<string, unknown> = {}): MailProviderConfig {
   const built = createMailProviderConfig(
     {
@@ -355,3 +438,94 @@ describe.sequential("live SMTP transport", () => {
     expect(harness.captured).toHaveLength(1);
   });
 });
+
+describe("SMTP client identity on the wire", () => {
+  /**
+   * The regression guard for a real production defect.
+   *
+   * A deployment relaying through an IP-authorized relay could not send at all: every attempt
+   * surfaced as `MAIL_PROVIDER_UNAVAILABLE`. The relay was healthy — `swaks` sent from the same
+   * machine — and the cause was the greeting. An SMTP client identifies itself in `EHLO`, and with
+   * nothing configured Nodemailer derives that from the machine hostname, substituting the address
+   * literal `[127.0.0.1]` whenever the hostname is not fully qualified. The relay closed the
+   * connection on it with a `421`.
+   *
+   * These tests are written against the wire rather than against the transport options, because the
+   * defect was invisible in the options: everything about the configuration looked correct.
+   */
+  it("greets with the configured name, exactly as configured", async () => {
+    const harness = await startSmtpServer();
+    const mail = provider(config(harness.port, { heloName: "mail.example.org" }));
+
+    await mail.send(
+      validateMailMessage({ subject: "Greeting", text: "Body.", to: "person@example.org" }),
+    );
+
+    expect(harness.captured[0]?.greetedAs).toBe("mail.example.org");
+  });
+
+  it("never greets as the address literal when a name is configured", async () => {
+    const harness = await startSmtpServer();
+    const mail = provider(config(harness.port, { heloName: "mail.example.org" }));
+
+    await mail.send(
+      validateMailMessage({ subject: "Greeting", text: "Body.", to: "person@example.org" }),
+    );
+
+    // The exact greeting the relay refused.
+    expect(harness.captured[0]?.greetedAs).not.toBe("[127.0.0.1]");
+  });
+
+  it("delivers to a relay that demands a fully qualified greeting", async () => {
+    const harness = await startSmtpServer({ requireGreeting: "mail.example.org" });
+    const mail = provider(config(harness.port, { heloName: "mail.example.org" }));
+
+    await mail.send(
+      validateMailMessage({ subject: "Accepted", text: "Body.", to: "person@example.org" }),
+    );
+
+    expect(harness.captured).toHaveLength(1);
+    expect(harness.captured[0]?.greetedAs).toBe("mail.example.org");
+  });
+
+  it("is refused by that same relay when no name is configured", async () => {
+    const harness = await startGreetingStrictRelay("mail.example.org");
+    const mail = provider(config(harness.port));
+
+    // Deterministic regardless of what this machine is called: the server accepts exactly one name,
+    // and Nodemailer's derivation cannot produce `mail.example.org` on any host.
+    await expect(
+      mail.send(validateMailMessage({ subject: "Refused", text: "Body.", to: "p@example.org" })),
+    ).rejects.toMatchObject({ code: "MAIL_PROVIDER_UNAVAILABLE" });
+    expect(harness.captured).toHaveLength(0);
+  });
+
+  it("carries the refused greeting into no error a caller can see", async () => {
+    const harness = await startGreetingStrictRelay("mail.example.org");
+    const mail = provider(config(harness.port));
+
+    const failure = await mail
+      .send(validateMailMessage({ subject: "Refused", text: "Body.", to: "p@example.org" }))
+      .catch((error: unknown) => error);
+
+    // A `421` is SMTP's "try later", so it stays `MAIL_PROVIDER_UNAVAILABLE` rather than becoming a
+    // permanent rejection — and the server's response text never reaches the caller.
+    const serialized = `${(failure as Error).message} ${(failure as Error).stack ?? ""}`;
+    expect(serialized).not.toContain("Try again later");
+    expect(serialized).not.toContain("closing connection");
+  });
+
+  it("still verifies through the same greeting Doctor's probe will use", async () => {
+    const harness = await startGreetingStrictRelay("mail.example.org");
+    const configured = provider(config(harness.port, { heloName: "mail.example.org" }));
+    const unconfigured = provider(config(harness.port));
+
+    // Doctor builds its provider from the identical validated configuration, so an operator whose
+    // relay refuses the greeting sees it in `pnpm doctor` rather than only when a reset email
+    // silently fails to arrive.
+    await expect(configured.verify()).resolves.toBeUndefined();
+    await expect(unconfigured.verify()).rejects.toMatchObject({
+      code: "MAIL_PROVIDER_UNAVAILABLE",
+    });
+  });
+});
diff --git a/docs/development/email.md b/docs/development/email.md
index 60e161a..c28973f 100644
--- a/docs/development/email.md
+++ b/docs/development/email.md
@@ -22,6 +22,7 @@ a new account or recover a password, because both need a message to arrive. See
 | `EKKLESIA_SMTP_USERNAME` / `EKKLESIA_SMTP_PASSWORD` | Optional **as a pair**. Half a pair is refused. |
 | `EKKLESIA_SMTP_FROM_ADDRESS` | Required once a host is set. The only address this platform can send as. |
 | `EKKLESIA_SMTP_FROM_NAME` | Optional display name. |
+| `EKKLESIA_SMTP_HELO_NAME` | Optional. The name this client announces in `EHLO`. See [When a relay refuses the connection](#when-a-relay-refuses-the-connection). |
 
 ### Security modes are explicit
 
@@ -36,6 +37,51 @@ plaintext session waiting to happen. Naming the mode removes the ambiguity:
 
 Certificate verification is always on and there is no setting that turns it off.
 
+### When a relay refuses the connection
+
+If a relay closes the connection before accepting anything — a `421`, or an
+`MAIL_PROVIDER_UNAVAILABLE` from a relay you can reach perfectly well with another client — suspect
+the **greeting** before suspecting the network.
+
+An SMTP client has to identify itself in the `EHLO` command, and Ekklesia does not invent that name.
+With nothing configured, Nodemailer derives it from the machine's own hostname, and substitutes the
+address literal `[127.0.0.1]` whenever that hostname is not fully qualified — which is the normal
+state of a container, or of a VM nobody has named. A relay that requires a resolvable client identity
+rejects that greeting outright, and because the rejection lands before any message is offered it is
+indistinguishable from the relay being down.
+
+This was a real deployment defect, and a generic one: the relay worked, `swaks` sent from the same
+machine, and only the greeting differed.
+
+```text
+C: EHLO [127.0.0.1]
+S: 421-4.7.0 Try again later, closing connection. (EHLO)
+```
+
+Set the name explicitly:
+
+```bash
+EKKLESIA_SMTP_HELO_NAME=example.org
+```
+
+```text
+C: EHLO example.org
+S: 250 ...
+```
+
+Use a fully qualified domain name the deployment owns, typically its sending domain. The value is
+validated at startup: no scheme, no port, no brackets, no whitespace, no trailing dot, and at least
+one label plus a domain — a single unqualified name such as `localhost` is refused, because that is
+exactly the input Nodemailer already discards.
+
+It is **not** a credential. It travels in the clear on every connection, so it appears in
+`pnpm doctor` output where an operator can check it against what the relay expects.
+
+Leaving it unset preserves Nodemailer's own behaviour exactly. It is deliberately not defaulted to
+the sender's domain: a machine whose hostname *is* a valid FQDN may already be greeting correctly —
+some relays check the greeting against the connecting address's reverse DNS — and silently replacing
+a working identity would trade one deployment's bug for another's.
+
 ### Google Workspace SMTP Relay
 
 The expected first production deployment, and a compatibility target rather than a dependency — no
@@ -58,6 +104,11 @@ controls which envelope senders are accepted.
 deployment relies on IP authorization, the server's egress address has to be registered, and a
 change of hosting provider or NAT address will break sending until it is updated.
 
+Such a relay is also likely to require a fully qualified `EHLO` name — this is the deployment that
+exposed the generic issue described in
+[When a relay refuses the connection](#when-a-relay-refuses-the-connection) — so
+`EKKLESIA_SMTP_HELO_NAME` is usually needed alongside the variables above.
+
 The same variables address Microsoft 365, a transactional provider's SMTP endpoint, a local Postfix,
 or anything else that speaks the protocol. Switching between them is configuration, not code.
 
diff --git a/docs/development/repository-workflow.md b/docs/development/repository-workflow.md
index e1a1442..7525aad 100644
--- a/docs/development/repository-workflow.md
+++ b/docs/development/repository-workflow.md
@@ -99,7 +99,10 @@ Doctor reports a fixed, deterministic set of checks:
   logical bucket are reported separately, because binding is not health.
 - Outbound email: skipped when no `EKKLESIA_SMTP_HOST` is set, a blocking failure when the
   `EKKLESIA_SMTP_*` settings are structurally invalid, and otherwise a connection that authenticates
-  where credentials are set and **sends nothing**.
+  where credentials are set and **sends nothing**. That probe performs the same `EHLO` greeting the
+  runtime performs, so a relay that refuses the greeting is reported here rather than discovered when
+  a message silently fails to arrive — see
+  [Outbound email](email.md#when-a-relay-refuses-the-connection).
 
 No check prints a credential: not the storage access or secret key, not the SMTP password, not the
 identity secret, not a module secret value.
diff --git a/docs/development/security.md b/docs/development/security.md
index 86642ec..d627ff5 100644
--- a/docs/development/security.md
+++ b/docs/development/security.md
@@ -614,6 +614,10 @@ becoming a liability:
   disables it.
 - **The password is a deployment secret.** Never in PostgreSQL, never in module configuration, never
   in an API response, never in Doctor output, never in an error, and no value in `.env.example`.
+- **The SMTP client identity is not a secret.** `EKKLESIA_SMTP_HELO_NAME` is announced in the clear
+  in `EHLO` on every connection, so it is shown in Doctor output where an operator can compare it
+  with what the relay expects. It is validated as a fully qualified domain name — no scheme, port,
+  brackets, whitespace, or control characters — so nothing arbitrary reaches the wire.
 - **Nothing is logged.** The transport logs nothing at all. Transactional mail carries verification
   and reset links, which are exactly the credentials those flows depend on.
 - **No blind retry.** One bounded attempt. A failure after the message data has been sent is