smtpsrv: Fix error code on transient authentication issues

When we can't authenticate due to a transient issue, for example if we
rely on Dovecot and it is not responding, we should use a differentiated
error code to avoid confusing users.

However, today we return the same error code as when the user enters the
wrong password, which could confuse users as their MUA might think their
credentials are no longer valid.

This patch fixes the issue by returning a differentiated error code in
that case, as per RFC 4954.

Thanks to Max Mazurov (fox.cpp@disroot.org) for reporting this problem.

internal/smtpsrv/conn.go

@@ -1004,9 +1004,12 @@ func (c *Conn) AUTH(params string) (code int, msg string) {
 		return 501, fmt.Sprintf("5.5.2 Error decoding AUTH response: %v", err)
 	}
 
+	// https://tools.ietf.org/html/rfc4954#section-6
 	authOk, err := c.authr.Authenticate(user, domain, passwd)
 	if err != nil {
 		c.tr.Errorf("error authenticating %q@%q: %v", user, domain, err)
+		maillog.Auth(c.conn.RemoteAddr(), user+"@"+domain, false)
+		return 454, "4.7.0 Temporary authentication failure"
 	}
 	if authOk {
 		c.authUser = user