Hello. I installed postfix & courier as this HOWTO describes. I also installed Horde/IMP and configured it to use courier at localhost as an IMAP server. I added domains & users to the MySQL table, however, I kept getting authentication errors when logging into Horde. In /var/log/maillog, there were error messages like:

Mar 17 23:33:53 hostname imapd: chdir example.com/webmaster/: No such file or directory

(I've changed the hostname and domain name in the example above, but otherwise that is a cut-and-paste from my log file).

Turns out, the solution was that the directory it's looking for in /home/vhosts isn't created until after the account receives it's first email. It wasn't until after I'd sent a test message (via smtp a.k.a. port 25) to webmaster@example.com that IMAP logins were sucessful.

Hope this saves somebody some grief.