This packages on squeeze and debian have a solution to get traffic information.
Indepent you using courrier or dovecot.
This have a couple of daemons looking the syslog can get all traffic info on realtime. The work now is discovery how it´s works and adapt it for ispconfig3.
I belive need create a little bit of adtional tables and change a couple of mysql queries.
Im easy backported the packages and used it on past part of DTC project.
Adtional will be gain a FTP Trafic information, IMAP Trafic great benefits.
Have way to build packages for redhat/debian easy. just need change the names and codes and release a special fork for ispconfig3.