golfguy,
Does your SMTP server changes the sender's address? And is this address fake? If that's the case, it could be why it's returning this error. Double-check that you're not changing the sender's address, or that this address is valid. Try using your real address or any valid ones, see if that works.
If nothing changes the sender's address, then maybe we'll need to connect to your machine to troubleshoot this problem further. In this case, I suggest you open a support issue. On our main web page, go to Services->Support->Report an Issue, and you'll be able to get assistance from one of our technicians.
Regards,
Rapha