[Dclug] Mail server recommendations

[Benjamin Urban]

I'm looking for a mail server solution with the following requirements:
* Must support IMAP
* Must support multiple domains
* Must be easy and relatively quick to set up (preferably without  
much scripting)
* Must be configurable and maintainable via a GUI
* Should support per-user trainable adaptive spam filtering (probably  
using IMAP folders)
I would prefer a FLOSS solution; failing that, a free (as in price)  
solution; failing that, a cheap solution, etc.  Years ago, I tried  
using Gentoo for this, but it failed at least two of those  
requirements; I'm looking for recommendations for distributions as  
well as the mail server software itself.