Security Alert - Non-delivery report looks like you are sending spam

Spammers, who do not want to send spam from themselves, often spoof the sending address to disguise who sent it.   If you are receiving non-delivery reports (NDRs) that look like you sent spam to someone, then the spammers are using your address. When the spam that appears to originate from you gets sent to addresses that don't exist, then the receiving mail server sends a non-delivery report back to the address where it thinks the mail came from: Your mail server.

Other spammers purposely send spam to non-existent email addresses with your address as the return address knowing that it will be delivered to you through a non-delivery report. Your spam filter will not block it because it appears to be a valid report, even though it is spam.

It's just like someone sending out a ton of junk mail through the US Postal Service with your return address on it. When the postal worker finds they can't deliver some of it, they return it to you.

Worldwide, there are billions of spam messages sent each day by perhaps millions of spam bots all over the globe.  Over 90% of all email is spam.  So, this is a huge problem.

I've had this happen to me as well from time to time.  These non-delivery reports occur for a one or two week spurt, and then they stop because the spammer stopped using my address. We can block all Non-deliver Reports, but then legitimate reports will also be blocked, so we don’t advise this as a solution.

My advice is to delete the messages when they arrive, just as you would the junk mail recieved through the US Postal Service.  It's not worth the time or effort to get upset about it or try and trace down who sent it.