Why am I receiving bounce-back notices for messages I did not send?
This happens to almost everyone at some point.
When Internet Mail (smtp) was invented, the internet was a much kinder/gentler place. One of its key weaknesses is that the from address of an email can easily be faked. This is called “spoofing” or “joe-jobbing”.
So, a spammer will send out messages with your from address, and if the receiving mail server can’t deliver it, it returns the email back to the sender. But, because of this spoof, the receiving mail server thinks that you sent the message, so you get the undeliverable message (aka bounce-back).
There is nothing that can be done to stop spammers from doing this. The good news is that eventually they will stop using your from address and will move onto some other victim.
You just have to wait it out.