We have 2 IronPort email security appliances at our gateway which handle our mail hygiene. We also have a GFI mail essentials/Mailsecurity box as the next inbound hop before hitting our Exchange 2003 environment. We have multiple applications that were hard coded to use the IP address of thesecondary box as a relay so unfortunately I cannot take out the secondary serverwithout creating mail flow problems. My question is when we upgrade to Exchange 2007 SP1, could I deploy an Edge server in place of the secondary server and have it use one of the IronPort servers as a smart host? Any help would be appreciated. Thanks!

Yes, you would configure this via a receive connector. http://technet.microsoft.com/en-us/library/bb123883(EXCHG.80).aspx