See also the Email Tracking page.
It's really all too easy to spoof/forge/fake just about every aspect of email. Doing so is illegal in some parts of the world. Spammers forge emails all the time, primarily because they know that what they are doing is illegal, so they want to draw attention away from themselves.
Frequently asked questions about email forgery
Why am I getting all these "bounce" messages about email I never sent in the first place?
A spammer somewhere has probably sent out a whole load of spam with a fake sender address - i.e. yours. In other words, when they sent their spam, they asked for any bounce messages to be sent to you.
OK, so what can I do about it?
The short answer is: not a lot. The slightly longer answer is as follows:
- You might be able to track down the spammer, complain to their ISP, and get them shut down. Only recommended for experts, and experts with a fair bit of time on their hands at that.
You can publish SPF records (again, only really recommended for experts, and possibly only if you also don't mind things breaking from time to time).
You can ignore, delete or filter out the bounce messages.