Roundcube probably has the best interface at the moment. With a little bit of coding, you could write a page to deal with user signups, and a little admin interface for you to deal with the account management.
Most MTA's have infomation on how you can add a footer to outgoing messages, so you could also provide them with pop and smtp mail.
Note to self: Add something funny! Search is your friend!
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Having seen the software in action, Socketmail seems to have many nice features... Mydocuments and allow to share folders etc...you can do multi-domain hosting (pro version) or multi-domain aliasing like mail.com does (using their Lite version)..You can do upselling such as offering premium account for $$ for certain quota size etc..
Something worth the money...
Atmail is just expensive for something that may be (from my view) less than what socketmail could offer..
Roundcube.. good for basic need, thats why its free..nice feature it has some ajax screens...drag drop stuff... but Socketmail is also developing ajax skin like atmail have...