Ah, yes–it's a bank. That's a great point. Not sure what I was thinking. I imagine you could set up a separate transport with special rules that dumps them somewhere outside of the normal mail infrastructure for that message thread but it wouldn't be worth it when the problem happens infrequently and just adding sender restrictions to the list is quick and effective.

It's probably Exchange so nuking the offending messages in the mail queues on the transports during a mail storm is par for the course. Even if they're distributed. The messages already hitting the mailbox hosts would definitely be causing a bad time though.