[PATCH 5/5] submission-login: submission-proxy - Recognize 421 4.7.0 as connection...

[PATCH 5/5] submission-login: submission-proxy - Recognize 421 4.7.0 as connection-limit reply

When the backend signals that the user's connection limit has been
reached, do not classify the response as a generic temporary
authentication failure (which causes the proxy to reconnect). Map it
instead to LOGIN_PROXY_FAILURE_TYPE_AUTH_LIMIT_REACHED_REPLIED, which
suppresses retries and reports the failure as proxy_dest_connection_limit
in the login_aborted event/log line. This mirrors what imap-login does
for the [LIMIT] response code and pop3-login for [IN-USE].

Other 421 replies (typically server shutdown / fatal error) are mapped
to LOGIN_PROXY_FAILURE_TYPE_AUTH_REPLIED rather than AUTH_TEMPFAIL: 421
means "closing transmission channel" (RFC 5321 Section 4.2.1), so a
reconnect on the same destination is unlikely to help and should not
happen automatically.

Gbp-Pq: Name 0005-submission-login-submission-proxy-Recognize-421-4.7..patch