I'm looking for solution for a specific problem: I want to setup a good and secure mail server but I need ipv4 + ipv6, some kind of autouser mapping (if I add new user to machine, system should automatically create a mailbox for added user with the same credentials which user uses for ssh) and ability to create custom mailbox (maybe stored in MySQL somehow) without having to add user do machine (email@example.com). Also ability to use clients like Thunderbird via POP and SMTP (optionally additionally IMAP) is crucial.
Any suggestions which approach/system should I use? Links are more than welcome. Much appreciated.