Unscrupulous people are playing this game to get your domains. This is a ploy I see happening more and more now as a way to talk you out of your domains. If you get an email complaining of spam from your domain, don't believe it without investigating further. Here's their method:
1) Recruit a bunch of friends, maybe a dozen.
2) All of you send them emails or messages or calls asking nicely for them to quit sending spam from that domain.
DO NOT answer any questions, Do NOT volunteer any information.
3) Inquire about purchasing the domain.
Leave it up to them to know what's going on with the domain, hopefully they're just letting it sit there & maybe they'll get the idea someone hijacked their domain & would like to sell it before things get ruff.
If they aren't interested in selling, they're might be spamming from the domain & have ruined it already.
If they bring up the fact that they are getting notices about spamming, & want to know if you're sure you want to buy, just buy it, then point them to this thread, but it's best if you point them to the thread after you have a bill of sale.