Route Internal exchange email through 3rd party spam filter (XEAMS)

My impulse is to say both it can't be done (although I'm not familiar with the specific product you're using) and that it's unnecessary.  What would be the particular reason that you want to route your internal email this way?  Surely you don't think that internal users are sending spam to each other? If you're thinking that it will help prevent malware infections on your internal workstations from causing the workstation to send out spam inadvertently, then the easiest way to do this is to set your perimeter firewall rules to prevent any SMTP traffic from going out except from the Exchange server and the XEAMS proxy server.

hypercat - two reasons. XEAMS has a nice built in email archive and I am need to retire my old mailarchiva server and the outbreak monitoring.  My users don't intentionally spam, but there have been a couple of cases in the last year where they became infected and started spamming.  In both cases, the evidence was in the users Outlook sent items folder.  Meaning the spam went through outlook and exchange.  Naturally, my firewall disallows any smtp traffic from leaving my network unless it comes from my server.  Also, in both cases, the spams generated went to local and remote recipients.  My current setup protects external recipients from any internal outbreaks generating spam with the XEAMS server in place, but i would like to protect my internal users as well.

In addition to Simon's comments, most of the big antivirus/antispam software vendors (Symantec, Vipre, etc.) have components that link in to Exchange on the server and Outlook on the workstation.  The ideal is to have multiple lines of defense.  If you don't already have this in place, you should seriously consider implementing it in your environment.