When I was system administrator back in 1999 one of the products that I used to work with on the daily basis was Exchange 5.5, I still remember the main components of the network that I used to manage: Windows NT 4, Proxy 2 and Exchange 5.5. I worked with Exchange Server from the version 5.0 to 2003, when 2007 was launched I was already 100% focus on ISA and didn’t have time to explore some features besides the integration with ISA Server. Now with TMG I will have a chance to start dealing with Exchange again to make sure that the SMTP Protection is correctly configured and also to troubleshooting possible mailflow issues (really fun!).
The new Email Protection on TMG is based on Exchange Edge and Forefront Security for Exchange and yesterday we just released a new article for the Tales from the Edge community site that has more details about how this works. Check it out at: http://technet.microsoft.com/en-us/library/ee338733.aspx