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.

All dependencies listed (in correct order) below are available at SBo:
1. perl-Sub-Uplevel (Sub::Uplevel)
2. perl-Test-Exception (Test::Exception)
3. perl-Carp-Clan (Carp::Clan) 
4. perl-Bit-Bector (Bit::Vector)
5. perl-Date-Calc (Date::Calc)

BETA VERSION WARNING: version 1.1.2 is in beta testing right now but has
an important bug-fix not found in production version (1.1.1).