I'm not a huge fan of MailEnable's webmail - I think it could have been done so much better.

hMailServer is working fine for me. I'm running it on MSSQL, too - which I think is a plus not having to install MySQL.

Exchange works well but you really have to know what you're doing to administrate it. (I clearly don't! Whoever left me in charge of Active Directory was a fool! Developers + Administration tasks do not mix.)