Roundcube

I'm trying to use this as my email server for State Transition and I will probably use it for client email systems.

Best setup instructions so far: https://help.ubuntu.com/community/Roundcube

But...that doesn't create any users, so I'm still a bit confused.

http://flurdy.com/docs/postfix/

tasksel installs dovecot and postfix. Dovecot handles IMAP/POP3 for retrieval and storage. Postfix is a mail transfer agent.