This is a challenge to overcome in BizTalk 2004. It seems that the message never hits the Send Pipeline and as far as I can tell no error is written to HAT. The only place this error is written to is the Event Log. But this is ok if you are using MOM or something else to watch the event log.
The messages will be Suspended inside HAT and marked as resumable.
CRITICAL: Note that this is not the case if a map fails on the Receive Port. These messages are non-resumable.