One spamming method involves sending email to you in a way so that it looks as though it came from your own domain. Spammers use this method as it is one way to have a spam message immediately whitelisted and bypass any filtering for your account.
The only way to prevent receipt of mail like this would be to set up graylisting on your account.
You can do this by logging into the cPanel administration account, at www.(yourdomain)/cpanel
Then click on BoxTrapper under Mail.
Then click on Manage next to the email address for which you’d like to configure the filtering, white/gray/black-listing.