First off, thanks for such a great product. I've been running Ability Mail Server for the past 2.5 years and it's run flawlessly!
I'd like to make a few suggestions that would make this even better:
1) For the Web interface, allow the user to specify how many e-mails to show per page. On a large monitor, 25 is too short a list (especially when wading through 300+ messages).
2) Under Filters and Spam, allow more than 5 rules to be added.
3) Add an option under the web interface, to reply as HTML e-mail. Otherwise, replying to an HTML e-mail defaults to text and the changing that to HTML wipes out the previous e-mail.
4) Last of all, the ability to specify alternating row colors in the web interface would greatly increase the readability.