Calling External Limit
The traffic rule for MRSA should be "outbound" as the traffic flow accros the TMG is from "internal to external" i.e the traffic is going out of the TMG into the DMZ.
I tried the rule with "outbound" configuration but the TMG droped my connections. When I configured the protocol "inbound" everything work fine.
The networks on TMG are configured with route relations, not NAT. I think that's make a difference.
Have you tried this scenario?