Count number of failed deliveries in mtail

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Postfix smtp (journalctl -u [email protected] -t postfix/smtp) should log something with status=bounced every time a mail is rejected.

For example:

status=bounced (host mailchat.pl[95.216.211.166] said: 554 5.7.1 No valid DKIM signature found (in reply to end of DATA command))

Such lines can be counted in mtail to see if the number of rejection grows for some reason. This should help with monitoring if mails are getting rejected because of blocklists. Of course most servers just silently consume what they think is spam and return no error.