Logged In: YES 
user_id=6405

That's very, very odd. Something in the chain is failing silently, and 
looking at the roundup-mailgw code I can't see anywhere that I could 
do that. Ultimately if that code broke, the MTA would report an 
undeliverable error. 
 
I can't reproduce the error locally - I think we'll have to add some 
logging to your mailgw. Are you up for that?