not long after I started where I work now (back in 98) we had a not so nice mail loop. We were a Novell groupwise shop at the time. A user left and set a forward to their new email address at a Uni and then a forward back from that account to there still active account at our site! Oh the joy email totally bombed for days and as was quite common back then Groupwise sat on our main Netware server meaning that users couldn't login either as all the servers resources were taken by groupwise.