You need: (horizontal) in order of preference:
an MTA courier, exim, postfix, (sendmail if you want to live on the edge.)
an MDA courier, exim, postfix, procmail
an IMAP server courier, some pop/imap server
a virus/spam scanner: amavis + spamassassin
+ a virusscanner of your choice.
in case of webmail: courier, squirrelmail.
Outlook will work with an imap server.
For courier you need to configure with Maildir's to support imap.
The courier mta has everything you need, RPM's are easy to make if needed.
Another solution is clark connect, it is installed within an hour, it is based on RedHat 9, web managed et al. (courier-imap, postfix amavis )
kind regards,