There are variety of solutions that would fit your needs. If you’re strictly looking for email and you have a tight budget, I suggest you purchase a POP server, put it behind your firewall, assign a public address to it and change your DNS MX record to point to it. Then you can point your Outlook clients to the new server.
If you need more than email, like collaboration, etc., and money is not a big issue, then I would suggest Microsoft Exchange 2000. You can still need to put it behind your firewall, make it public, etc.