Ah, mailing loops are great.

I remember working for an ISP in the mid 90s. We never really had problems with 1 to 1 mailing loops bouncing back and forth, but we ended up with a large circular mailing loop involving a mailing list, and bad addresses on it getting bounced to the previous server which sent a reply to the mailing list, which got bounced and sent to everyone in the group which caused someone else's mailbox to fill up that was in a forward, which for some reason sent a bounce to the mailing list that really started to set off the explosive growth.

Needless to say the bounces seemed to be growing quadratically and overwhelmed our medium sized ISP, a decent sized college, and a large ISPs mailing system in less time than anyone could figure out how to get it to stop.