Note: The following is part of a series of steps to setup an email server using Exim 4.x, with imap and webmail access. It will use winbind to get user information from an NT server. If you found this page via a search engine it may not cover what you need or you may need to start at the beginning to understand everything I have done.
I started with UW, but decided to give Courier a try. I think Courier will be my choice because it stores emails as individual files (maildir format). My business gets a lot of large attachments and I would hate to think what it would take to delete some of those from a flat file. I'm leaving both IMAP setups for reference.
A comparison of Courier and UW (more so than of mbox vs maildir imho): http://www.courier-mta.org/mbox-vs-maildir/
Cyrus is another option, but I decided for various reasons not to use it.