Most flexible MTA around. excellent smtp handling and restriction functions, support for ssl, cyrus, ldap and whatever you need.
with its content filter you can do anithing you want.
but you need some addon pop/imap/http server part for user access.
check out this doc, besides postfix.org homepage.
http://workaround.org/articles/ispmail-sarge/
should be exact answer.
i followed this doc and did it, without any previous understanding about mysql
on debian box.
only sqwebmail is quit simple, you may wish more userside http functions.
if it will be your first postfix, reading whole postfix description and main.conf parameters is must. better you know it, more you love it.