There may be time when you would like to grab bounced mails from the logs which you might have given to someone but forgot to create as --add-mailalias in YunoHost.
In other words if the mail gets bounced because the email id in not linked to any account, you can grab them by this command:
$ grep 'User unknown in virtual mailbox table' /var/log/mail.log | sed -e 's/.*to=<//g' | sed -e 's/>.*//g' | sort -t: -u -k1,1
This is important if you are missing some mails because you thought that mail is registered to your server but actually its not.