Command:

tail /usr/local/psa/var/log/maillog -n10000 | grep "user@example.com"

Notes:

Better to tail than cat (in case its a big log)

-n10000 is going back 10000 lines

Add -f if you want to see a live feed of the log

tail -f /usr/local/psa/var/log/maillog | grep "user@example.com"

Source: http://www.woyano.com/view/578/How-to-grep-a-mail-log-to-see-if-a-message-bounced