Spam, from myself?: “Ok, so this morning I’m checking my CrabApples.net email and notice a spam from my own address. How in the world do I stop that?”
Poor Rob, this has happened to my father before now as well.
One day he received hundreds of emails from various domains saying that there was no such user “email@example.com”. These bounces were being sent to him as the return address in some spam email sent to “somedomain.com” was set to a random address on his domain.
The first step I took was to remove the catch-all rule for his domain, and the emails bounces stopped. They may still be spamming using his domain using SMTP relaying, but no bounce emails are coming back. It is good practice to not have a “catch-all” rule for your mail domain, and it is also prudent to have a “postmaster” email account, as some domains will not accept email from you without one.