I would change that Courier or any Dovecot with a Cyrus IMAP implementation... it really rocks in terms of stability (and works smoothly), is fast and replication :)
Really? My experience has been the complete opposite.
Almost every time I've even been involved with or ran any medium/large mail infrastructures (200K active mailboxes) I've always found Dovecot to be a better choice.
Good to hear that Dovecot scales. I personally went with Cyrus because at the time I was picking an IMAP server, Cyrus was already a well tested solution known to scale. As I recall, Dovecot back then was still young and developed by one person. I've heard a lot of good things about it since though. I'm sticking with Cyrus since it works fine for me, but it's good to have options.
