What do you do? Nothing. There’s nothing you can do.
Why is it happening? In a word: spammers. Let’s look at what they’re up to.
Become a Patron of Ask Leo! and go ad-free!
Spam often appears to have come from someone who did not send it at all. If that’s you, then you may get bounce messages when that spam is identified as spam by its recipient, or when it was sent to invalid email addresses. There’s nothing to be done, as it was never your doing to begin with.
Why did I get a bounce message?
Bounces for emails you didn’t send are the result of spammers trying to get people to open their spam and click on the links in the spam message.
Spammers want their email to look like it’s coming from someone who might be trusted. In other words, they want it to look like it’s coming from you. “From spoofing” allows them to do exactly that. They write messages using your email address, and maybe even your name in the “From:” address. It’s easy to do, and spammers have been doing it for a long time.
If the email address the spammer sent it to belongs to a real person, they may see email looking like it came from you. They probably have no idea who you are, and perhaps mark it as spam.
Since spammers are simply blasting email out to huge databases of email addresses, they have no idea whether those addresses are legitimate or not. If they send email to an address that no longer exists, never existed,1 or recognizes the message as spam, it bounces automatically.
The email delivery system says, “Hey, I can’t deliver this email; I’m going to return it to the sender.” Because the sender information on the “From:” line looks like it was you, you get the bounce message.
None of this happened on your machine or your account. In fact, it had nothing to do with you, other than your email address appearing in a spammer’s database.
Steps to take
Unfortunately, there’s nothing you can do to stop it.
All you can really do is mark them as spam and let your email program handle it from there.
If random strangers complain you’re sending them spam, you can point them at this article. As I said, you weren’t involved, it’s not your fault, and there’s nothing you can do.
However, if several of your friends complain you’re sending them spam, it might be something more. Take a look at “Someone’s sending from my email address; what can I do?” That article includes a couple of quick questions to determine whether other things, such as your account being compromised, have happened.
From the sound of your scenario — getting a bounce from someone you have never heard of from some address that you never sent email to — the best thing to do is mark it as spam and get on with your life.
If you found this article helpful, I'm sure you'll also love Confident Computing! My weekly email newsletter is full of articles that help you solve problems, stay safe, and give you more confidence with technology. Subscribe now and I'll see you there soon,
Footnotes & References
1: Often the result of a dictionary-style approach to generating email addresses to send to: email@example.com, firstname.lastname@example.org, email@example.com, and so on — whether or not those accounts actually exist.