Connecting to Exchange Server over the I

In an Exchange Tips Newsletter, there was a tip about having users connect to their mailbox over the internet using TCP/IP. The tip, however, did not go into detail about how to do this. Do I need to list the Fully Qualified Domain Name in the Exchange Server service tab? Any help would be appreciated. As it is setup now, the user get an error stating the Exchange server can not be found. Thanks much!! -Kris