The most likely explanation is that someone with your email addrees in their address book has been infected with a virus. Emails are now being sent with your address in the "From:" field, so that any which bounce will come back to you even though you never sent them originally.
There are also viruses that just send emails purporting to be "undeliverable" messages.
If you don't already have it, "Mailwasher" is a very good program for checking what mail is waiting on the ISP's server so that you can delete any junk/spam/viruses without first having to download them. click here for the "free" version.