There maybe times when someone would send an email to your server with wrong address which would bounce. As an system admin you should keep a track of these wrong mails and map them with probably correct mail address. This will help your users to get important emails which they otherwise would have missed.
Here is the grep command which gets the email address of the bounced address sent to your server.
$ 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
Your configuration maybe different, so may require to do little changes to it.