Someone is Using my Domain to Send Spam - What Can I do?

What is probably happening is that someone is forging the from header of the spam emails they are sending out. The from header in an email is actually fairly meaningless and can be easily forged. There is nothing you can do about this, other than publishing SPF (http://spf.pobox.com) records for your domain(s). If you would like a second opinion about any weird emails, feel free to send the full headers to Support. You can also email Support if you would like to publish SPF records for your domain and thereby help other mailservers identify and block forged email that uses your domain(s).