The most likely cause is from email spoofing.
E-mail spoofing is the forgery of an e-mail header so that the message appears to have originated from someone or somewhere other than the actual source. Distributors of spam often use spoofing in an attempt to get recipients to open, and possibly even respond to, their solicitations.
If you examine the email header information you will find that the IP of the email originator is not your IP and the email address will contain your domain name.
There is little we can do about email spoofing because it does not originate from our servers, anyone can spoof an email just by changing the from setting in there email client.
Our recommendations are to go into your cpanel - click on email - click on default address - set the default to :blackhole: - this will delete all unrouted emails. Next click on "Spam Assassin" - click on ENABLE Spam Assassin - click on DISABLE spam box - This will turn on your spam filters. after this you should see a drastic reduction in the amount of spam you receive.