As with others here, thanks. I'm using Debian 5.0 but it runs the same.

Note that is you are using rsyslog you'll need to comment out the mail.log reference in /etc/logrotate.d/rsyslog or you'll get a duplicate log reference error when logrotate attempts to execute. If you don't notice it for a couple days it could be a real bitch as it prevents logrotate from running.

Snip from /etc/logrotate.d/rsyslog:

...
/var/log/mail.info
/var/log/mail.warn
/var/log/mail.err
#/var/log/mail.log
/var/log/daemon.log
/var/log/kern.log
/var/log/auth.log
/var/log/user.log
...