SMTP email error

The attached message has failed delivery and has been referred
to you as postmaster. The following error report or reports
were given to explain the problem:
I keep getting this email error message:

— Problems not related to specific addresses in the message:
501 … Sender domain must exist
*** weatherby@ionet.net
I have looked at the attachment and it is the return read reciept. The orginal made it ok.
What is causing the 501 error?
Where canone find documentation on this type of error and what the cause might be?