Usually this question arises when you receive emails from your own domain name or returned emails that you never sent. Please note that this does NOT indicate that someone has access to your email account. Unfortunately anyone can send email as anyone else, you can try it right now, set your own reply to address to be firstname.lastname@example.org and send yourself an email. You will see that it arrives properly and looks like its from ebay. Spammers are trying to use your own domain name to make you believe that the email is real and to open it.
The most common question we get is how can this be stopped. To stop these types of emails do 2 things.
1. If you have a catch-all email account enabled delete it. This will reduce the number of mis-delivered emails that are returned to you to only those with real email addresses.
2. Make sure that your own email and/or domain name is not white listed. It is not necessary to white list yourself and this will cause all email addressed from your own domain name to be automatically accepted without any checks to see if it is valid. Spammers know that people do this and they take advantage of it.