I had one like that - I was getting shit that allegedly came from a North London firm I was in a BITTER piece of litigation with (I lost but also sort of won in that when I could not force the other side to reveal the notes I knew they had of some meetings with a dead person my client agreed to pay theirs a lot of money - and then went bust so they never got their money) and another firm in the West Country was also getting shit and I and the West Country firm's pointy-head went over the full headers - and eventually we found a little "licensed conveyancer" also in North London. He had a virus. It was sending messages and spoofing the sender field from all the incoming emails he had ever had - and the firm in North London had at one stage mistakenly sent an email to him with my address also on.

I thin the West Country firm's pointy-head helped the conveyancer get rid of the virus.