Ok, I was like WTF. Thanks for letting me know.
Don't worry, it's no biggie. happens to us like once each 6 months, most often in Spamhaus but in others too. But we don't do any outbound, just replies to customer inquiries. So nothing qualifies as spam. You can think of it as a glitch.
Make sure you use SPF at least. For example to block anyone from sending email, add a TXT record with this content: "v=spf1 -all" , without the quotes. you need to add 3 records like this, and in the host field enter: @ (root), then another for: www. Side note we also add a third for * (subdomains) to be 100% sure, if there are any.
Side note anyone managing their own email have to periodically do these things as part of maintaining email deliverability. If your domain is for sale, being in a blacklist is no problem. The new buyer simply delists it and unless they start doing spam, emails will work just fine.
Edit: "v=spf1 -all" means all emails are blocked and this domain isn't sending any. If you want to send some, you need to whitelist your server IP, for example: "v=spf1 +ip4:22.33.44.55 -all" will whitelist the IP=22.33.44.55 and everyone else is blocked.