Couriergraph is a very simple mail statistics RRDtool frontend for Postfix that produces daily, weekly, monthly and yearly graphs of Courier's POP3, IMAP, POP3s and IMAPs logins.
Assuming you already have Courier and Apache installed, just install these packages:
apt-get install rrdtool couriergraph
Navigate to this location to view the graphs:
http://yourserver/cgi-bin/couriergraph.cgi