As I said earlier and before in a previous post you need to setup either an
IMAP or a
POP3 server to allow mail retrieval for users.
SMTP only is not a complete mail solution , You need an
IMAP server such
Courier-IMAP or a
POP3 server such as
Dovecot-POP and you might need some database server such as
MYSQL and web a server such as
Apache to publish some
webmail client to allow users to send and view their emails via web browser also you can setup some Anti-Virus / Anti-spam software if needed .
Drop by
postfix.org and check their Howtos you will find what you need.
Now you have the required knowledge to setup your mail solution go ahead.