It would be news for me if LimeWire was illegal to use. The court action in the USA over LimeWire prevented the company distributing the software. But that is totally different to other persons using it for personal use.

The email sounds highly suspicious to me. I'd re-check what it says & who it claims to be from. I'd personally trace the source of their ip address.

Feel free to forward a copy to the forum's administrator email address if you like so it can be reviewed.