Multiple routes and ISA2004 spoofing block

We are in the process of moving from SBS2003 with ISA 2004 to TMG and are currently running both firewalls with seperate Internet connections.

Inbound SMTP traffic tru the TMG firewall works fine as long as the email appears to come from the internal TMG nic adress, If we enable "send originating IP adress" for logging purposes we will get spoofing detected.

Is there a way to get this working with ISA 2004?

TIA,

Fred