Could someone tell me what is the difference between cyrus and courier? I am trying to buuld an imap server to serve virtual users and to work with postfix.
Thanks,
Miles

You could start here..

http://en.wikipedia.org/wiki/Comparison_of_mail_servers
http://www.courier-mta.org/intro.html
http://cyrusimap.web.cmu.edu/generalinfo.html

Courier IMAP stores mail in Maildir format.
Cyrus IMAP stores all mail in a database.

Dovecot is another more recent popular solution to look at if you want a lightweight IMAP implementation.

I believe configuration complexity in order of Most difficult to setup to Easiest to setup would be as follows.
Cyrus - Most Difficult Config
Courier - Config not bad
Dovecot - Easiest configuration


I've been running Courier IMAP with Postfix for the past 5 years, and I've not had any issues with it. This is the setup I used most recently after a catastrophic hardware failure took out my original mailserver. http://www.debian-administration.org/articles/364



If you really want to know the deep down differences between these IMAP servers I would suggest The Book of IMAP