I have a model in which I have a very simple mail relay server in a DMZ, port 25, all it does is forwards mail to my Exchange Server 2007 internally. On the Exchange 2007 server, I am running a filtering software for Spam and other things that handles mail before passing to Exchange. Part of the flow, is that if a message is blacklisted (true ip in the header not my relay server) a response code in the 400's or 500's is returned to the Relay Server (I am not sure what the code is at this time). I have 2 questions - related to this model... 1) It appears that once the relay server gets the 4xx/5xx code in return, it barfs and all mail starts getting queued up - valid emails are not passed. Is there a way to configure SMTP in IIS to handle this kind of behavior? 2) If my plan is to move to Exchange 2010, how would I replace my Relay server? What component would I install on it - if I still want my full exchange server to be hidden behind the firewall... ? Thanks J Additional Info: The internal filter software is returning a 421 or a 550 back to the SMTP service (relay Server) apparently once that message is received all SMTP traffic stops not just for a specific message.

1) What do you see in the SMTP protocol logs on both the SMTP relay server and the Exchange server? That might give you a clue what's going on. 2) Exchange 2010 Edge role does that and numerous other features. Read all about it on Microsoft's Exchange site or search the Internet.Ed Crowley MVP "There are seldom good technological solutions to behavioral problems."