It's a nice o/s agnostic -- read: openBSD friendly -- web gui administrated post office. pop, imap, web client access. A file system structure that lends itself to simple (agent-less) backup techniques.
Also, the free-edition limit is on "mailboxs," not aliases. You may have many aliases to each mailbox. So those "four for [you]," may possibly be implementable as "one plus three aliases," which is net one (1) mailbox.
An example ...
john.smith@domain.com
-- jsmith
--administrator
--postmaster
Is one mailbox + three aliases.