docs: Document how to use SMTP without authentication.

This is the behaviour inherited from Django[^1].  While setting the
password to empty (`email_password = `) in
`/etc/zulip/zulip-secrets.conf` also would suffice, it's unclear what
the user would have been putting into `EMAIL_HOST_USER` in that
context.

Because we previously did not warn when `email_password` was not
present in `zulip-secrets.conf`, having the error message clarify the
correct configuration for disabling SMTP auth is important.

Fixes: #23938.

[^1]: https://docs.djangoproject.com/en/4.1/ref/settings/#std-setting-EMAIL_HOST_USER

docs/production/email.md

@@ -12,7 +12,9 @@ email addresses and send notifications.
 1. Fill out the section of `/etc/zulip/settings.py` headed "Outgoing
    email (SMTP) settings". This includes the hostname and typically
    the port to reach your SMTP provider, and the username to log in to
-   it. You'll also want to fill out the noreply email section.
+   it. If your SMTP server does not require authentication, leave
+   `EMAIL_HOST_USER` empty. You'll also want to fill out the noreply
+   email section.
 
 1. Put the password for the SMTP user account in
    `/etc/zulip/zulip-secrets.conf` by setting `email_password`. For

zerver/lib/send_email.py

@@ -599,5 +599,6 @@ def log_email_config_errors() -> None:
     """
     if settings.EMAIL_HOST_USER and settings.EMAIL_HOST_PASSWORD is None:
         logger.error(
-            "An SMTP username was set (EMAIL_HOST_USER), but password is unset (EMAIL_HOST_PASSWORD)."
+            "An SMTP username was set (EMAIL_HOST_USER), but password is unset (EMAIL_HOST_PASSWORD).  "
+            "To disable SMTP authentication, set EMAIL_HOST_USER to an empty string."
         )

zerver/tests/test_send_email.py

@@ -165,7 +165,8 @@ class TestSendEmail(ZulipTestCase):
             error_log.output,
             [
                 "ERROR:zulip.send_email:"
-                "An SMTP username was set (EMAIL_HOST_USER), but password is unset (EMAIL_HOST_PASSWORD)."
+                "An SMTP username was set (EMAIL_HOST_USER), but password is unset (EMAIL_HOST_PASSWORD).  "
+                "To disable SMTP authentication, set EMAIL_HOST_USER to an empty string."
             ],
         )
 

zproject/prod_settings_template.py

@@ -71,7 +71,9 @@ EXTERNAL_HOST = "zulip.example.com"
 ## advice for troubleshooting, see the Zulip documentation:
 ##   https://zulip.readthedocs.io/en/latest/production/email.html
 
-## EMAIL_HOST and EMAIL_HOST_USER are generally required.
+## EMAIL_HOST and EMAIL_HOST_USER are generally required.  If your
+## SMTP server does not require authentication, leave EMAIL_HOST_USER
+## commented out.
 # EMAIL_HOST = "smtp.example.com"
 # EMAIL_HOST_USER = ""