I know it's not exactly what you asked for, but did you know Google will host your domain email for free? They have both IMAP and POP3 access, and they will use your own domain name so basically nobody will know it's hosted by Google unless they check the ip address.
Just make an A record for mail.whateverdomain.com, point an MX record to Google's server and you're set. I do this for my domains and it's awesome (and free!). You also get a web interface (basically google's, though you can modify it a bit), but if you only give out IMAP info that's all that's needed.
Just sign-up for a Google Apps account here if it meets your needs :
Google Apps Standard Edition helps groups build communities - Google Apps Standard Edition