fix: let a deployment name itself in the SMTP greeting

A real deployment could not send mail through an IP-authorized relay.
Every attempt surfaced as MAIL_PROVIDER_UNAVAILABLE while the relay was
healthy and swaks sent from the same machine.

The cause was the greeting, and it is generic rather than tied to any
provider. 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, which is the normal state of a container. The relay
closed the connection on it with a 421 before any message was offered,
which is indistinguishable from the relay being down.

EKKLESIA_SMTP_HELO_NAME now lets the deployment say what to greet as. It
is validated as a fully qualified domain name — no scheme, port,
brackets, whitespace, or control characters, and at least one dot, since
a single unqualified label is exactly the input Nodemailer already
discards. It is not a credential: it travels in the clear on every
connection, so Doctor prints it where an operator can compare it against
what the relay expects, and Doctor's probe performs the same greeting the
runtime does.

Absent preserves Nodemailer's behaviour exactly. It is deliberately not
defaulted to the sender's domain: a machine whose hostname is already a
valid FQDN may be greeting correctly, and some relays check that against
reverse DNS, so replacing it silently would trade one deployment's bug
for another's.

The regression guard is a raw SMTP server that refuses any greeting but
the expected one, because smtp-server exposes no EHLO hook and its
earliest hook is never reached by a send-nothing probe.

Co-Authored-By: Claude Opus 5 <[email protected]>

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.