How can I stop someone from sending spam using my domain?

Hi! I have been getting bounced emails that appear to have been sent to the recipient from my domain. I use Google Apps to manage my email and my account doesn’t appear to have been compromised. Neither does my registrar account. The emails appear to be sent by a nonsense email address ” kEuHqXwrc@<myDomainName>.com.” So far, there have been 4 email addresses that have bounced, each are from nonsense email accounts. Any idea how I can shut this down? Is it some kind of scam to get me to click on some of the links? Any help would be greatly appreciated!