Where are the Mail logs stored

I am new to linux and I am running Postfix as my Mail server. Comcast customers are not receiving mail that is sent from my server. I need to check the logs. I can't seem to find them. Can somebody point me in the right direction.

thanks

Usually /var/log/maillog
grep it for comcast, and for the matches: grep the mailIDs, so that the view is consistent.

Maybe this will also help:
Postfix Debugging Howto

Also, it seems, that postfix by default uses syslog.
That´s why the logfile is defined in syslog.conf
Postfix Basic Configuration