Unfortunately, the second part of your question is actually the easier part.
What do you do? Nothing. You can’t do much about that.
But why is it happening? Well, that’s an interesting scenario.
Why did I get a bounce message?
When you get a bounce message for emails that you didn’t send, you simply need to mark them as spam and let your email program handle it from there. These bounce messages are a result of the techniques spammers use to try to get people to open their emails and click on the links in the message.
Spammers want the email to look like it’s coming from someone that might be trusted. In other words, they want it to look like it’s coming from you – and that’s exactly what they do. They write up messages and use your email address and maybe even your name in the From address. That’s easy to do and spammers have been doing it for an awfully long time.
If the email address that the spammer sent it to is a real person, then they may see email that looks like it came from you. They probably have no idea who you are and rightfully mark it as spam.
Sometimes, spammers get the address wrong. In fact, they’re simply blasting email out to addresses whether they know they work or not. So, if they send email to an address that doesn’t exist or perhaps an address that recognizes the message as spam, it bounces automatically.
In this case, the email delivery system 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 came from you, you get the bounce message to an email that you never sent.
You’re never part of this process. None of this happened on your machine or your account. In fact, it had nothing to do with you. And unfortunately, there’s nothing you can do to stop it.
When it becomes a problem
If you end up with people complaining that you are sending them spam, then you need to look into other things.
Take a look at an article that I have, “Someone’s sending from my email address; what can I do?” That will actually run you through a couple of quick questions to determine whether other things, such as your account being compromised, have happened.
But from the sounds of your scenario where you’re 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.