[PATCH 4/5] submission-login: client-authenticate - Reply 421 4.7.0 on mail_max_useri...

[PATCH 4/5] submission-login: client-authenticate - Reply 421 4.7.0 on mail_max_userip_connections

Until now the connection limit was reported via the same 454 4.7.0 reply
that is used for generic temporary authentication failures. That makes
proxies (including Dovecot's own submission proxy) treat the rejection as
a transient auth error and retry, which is futile when the limit is hit
and only obscures the actual cause in the proxy log.

Reply with 421 4.7.0 instead. RFC 5321 Section 4.2.1 specifies 421 as
"service shutting down, closing transmission channel", which is the
right signal for "do not retry on this connection". The 421 + 4.7.0
combination is unique among the 421 replies emitted by submission and is
used by the submission proxy to recognize this specifically as a
connection-limit reply rather than a generic 421 internal/shutdown.

Gbp-Pq: Name 0004-submission-login-client-authenticate-Reply-421-4.7.0.patch