[PvK]

Does the full header info not have more info about the actual originating machine?

Is dogscoff.plus.com even a domain name you use in any way?

You could put a monitor program on your net traffic to see if your machines are actually sending out any rogue email.

And/or ask your ISP to check into it.

My guess however is your name is just in someone's random database of names to spoof as, perhaps even from web search data. Some of us sometimes google you to find your site rather than looking up the URL, so web spider bots will dredge up your addresses and abuse them.

PvK