Source IP address for outgoing communications from the external interface of the TMG firewall is important in a number of scenarios. One of the most important of these scenarios is the TMG email policy scenario where you need to be sure that the correct IP address is used for outbound SMTP messages.
What do you do when the wrong IP address is being used?
Hit the TechNet wiki and read this article:
DEBRA LITTLEJOHN SHINDER
MVP (Enterprise Security)