It's a Phishing expedition. More than likely, if they're like the ones I've received, they give you an address to click on to obtain more information why your "e-mails" were returned. DON'T CLICK ON THOSE LINKS! Delete the e-mails, and if you can, put the from addresses on a black list.
This was something I went through some time ago and did just like you, checked my sent box. When I realized I hadn't sent an e-mail to that address, and that they wanted me to click on a link to obtain more information, the light went on and the e-mails were deleted.
If something gets your goat, it just proves you have a goat to get.