Many organizations have an internet connection with only 1 ip address.

This presents a problem - how do outside users relay mail thru the company email system?  The outside IP address points to the spam filter, not the exchange box, for SMTP.  As you can see in the network diagram linked above, if I setup any email client on the outside of my network, the spam filter will be responsible to handle the username & password that client provides for email relay.

This means we need to setup our spam filter to relay email for exchange users.

Install Video 7 - setup spam filter to relay for outside users - including blackberry phones and the Apple iPhone

Basically, you will edit /etc/sysconfig/saslauthd with Webmin, entering the IP address of your Exchange server with IMAP enabled.  That is all you need to do - edit one text file to enter a single IP address, restart a service with Webmin, and you are ready to rock.