pflogsumm provides summary of email traffic by parsing maillog files.
Some of the details provided by pflogsumm include:

Total number of messages Per-Day/Per-Hour/Per-Week/Per-Month traffic
summary.  Postfix error messages and much more.

For more info, please visit the the official website at:
http://jimsun.linxnet.com/postfix_contrib.html

Once package is installed, see man pflogsumm for detailed usage info
and pflogsumm.pl -help for a terse version.

Please see included sample_cronjob.txt file and manpage for details
on running pflogsumm via crontab and getting stats via email.

BETA VERSION WARNING: version 1.1.5 is in beta testing right now.  If you
are using postfix 2.9, you need this version as it contains fixes to
logging changes incorporated in postfix 2.9.