Most likely your email address was "spoofed" by a spammer, and used to send to a boatload of email addresses, some of which will be invalid. Ergo the daemon bounce message. If this is the case, there is nothing to do but wait it out. Spammers will usually use an address for a short period of time then move on to annoy someone else. You could change your password just to make sure the account hasn't actually been compromised. But if your Sent folder looks OK my money is on spoofing.