It would be good to see exactly what the packet is.
But is is likely that experts exchange was sending an email to you when your end perceived the session to be complete, whereas e-e's email server hadn't finished yet.
For example, if your email server sent a FIN, before the FIN-ACK arrived from e-e, TMG cleared the session from its state table, once the FIN-ACK actually arrived it didn't have a matching entry in the state table and so discarded the packet with the above error.
So you say every 50 seconds - for how long has this been going on? It would probably retry for a bit and then give up.
