I really like MBEQWA and I’ve used it successfully in my small e-mail systems in the past. MBEQWA basically gives you a Web console where you can add/remove/modify e-mail users, and even hosted e-mail domains. I had to modify it to work with my database layout, but it has worked wonderfully for 2+ years.
Unfortunately, it’s fallen out of development, and changes in PHP require modification to the package to make it function today.
I’ve e-mailed the author asking if I could help contribute. We’ll see if he’s interested.
Wish me luck!