Something like Postfix with an IMAP server - dovecot ot Courier-IMAP for example.
I use Postfix with dovecot, plus amavis-new, spamassassin and clamav for spam and virus filtering.
Outlook can connect to IMAP servers, and you can use squirrelmail or similar for web access
Have a look at
www.postfix.org
and there you'll find plenty of howtos
Bill