The current fail2ban regexp catches all SMTP connection errors.
This works fine, but includes connection errors, that can be caused by
transient external causes, and accidentally delay email delivery.
This patch changes the regexp to be more targeted towards specific SMTP
errors that are likely to be caused by deliberate actions.
The expression was cross-checked with a few month of errors to confirm
it should not have false positives, and that it correctly left
connection errors alone.
Thanks to pepperbob@github for reporting this in
https://github.com/albertito/chasquid/issues/77.
7d56f1b4b4