I haven't worked on windows platform, so can't help much. But, google gives out lots of results. Depends on what you want now, free or paid and also what types of features you require.
Some other important considerations.
Do you have static IP? Has the DNS been setup properly? especially the reverse lookup?
Do put proper spam filtering in place. Good firewall policy is quintessential, don't let your server turn to open relay on the internet for spammers.
Also, check if your IP isn't in any kind of blacklists or your mail might go to spam or worse be rejected by mail server.
And, if you have non-Outlook users using the server avoid MS Exchange as far as possible.
And it would be good to implement tls, imaps/pop3s if people will be connecting via pop3/imap to the server, especially over wireless connections.