If you experience mails not being delivered on Ubuntu 9.10, have a look into your /var/log/mail.log . Chances are good that you find a message like
courierfilter: /var/run/courier/courierfilter.pid: Permission denied
This is a bug in either the deb package process or in the run-scripts and already known to Ubuntu and the underlaying Debian.
As a quick fix you can adapt the permissions yourself:
chown daemon:daemon /var/run/courier chown daemon:daemon /var/run/courier/authdaemon