http://workaround.org/articles/ispmail-sarge
VPSs are nice but how do you host multiple domain email addresses? Well the above tutorial a fantastic guide on Debian Sarge.
An item worthy of note is creation of the Courier SSL certificates. By default there will be a couple automatically created by Courier at install time. This are useless as your email client WILL reject them because localhost is not the same domain as domain1.com etc…
You need to edit both /etc/courier/pop3d.cnf and /etc/courier/imapd.cnf and change the details to reflect your company or name.
CN – Fully qualified hostname (eg. mail.domain.com) C – Country (2 letter code eg. AU) ST – State or province L – Location/City O – Organisation OU – Organisation Unit/Division
CN must be set to your mail servers hostname!
Now you need to remove the old certificates because the new ones cannot be generated with them in place:
cd /etc/courier rm imapd.pem pop3d.pem
Now to make the new certificates, which will last for 365 days at which point you need to re-generate them. If you need them to last longer then you can adjust the /usr/sbin/mk*****cert shell script.
mkimapdcert mkpop3dcert