From LQWiki

Install the imap daemon as described under installing software. Start the imap daemon like this:

/etc/init.d/courier-imap start

Verify that it is running like this:

lvps87-230-12-146:/etc # nmap localhost | grep 143
143/tcp   open  imap

Configure the daemon via /etc/courier-imap/imapd. The only change I did was

AUTHMODULES="authpwd"

This line has the effect that /etc/passwd is used to authenticate the imap users.

TroubleShooting

login to server failed

If you get the error message "login to server failed", it is probably because your authentication daemon is not running. So, start it and restart courier

/etc/init.d/saslauthd restart
/etc/init.d/courier-imap restart