This article describes how you can monitor your Postfix mailserver with the tools Mailgraph and pflogsumm. Mailgraph creates daily, weekly, monthly, and yearly graphs of sent, received, bounced, and rejected emails and also of spam and viruses, if SpamAssassin and ClamAV are integrated into Postfix (e.g. using amavisd-new). These graphs can be accessed with a browser, whereas pflogsumm ("Postfix Log Entry Summarizer") can be used to send reports of Postfix activity per email.

http://www.howtoforge.com/postfix-monitoring-with-mailgraph-and-pflogsumm-on-debian-lenny