is the address:
the address you entered in the From field?
It would seam that the SMTP server is refusing to relay messages that are sent from a domain that differs from the domain the SMTP server is hosted on.
It may be that the SMTP server requires authentication to relay emails in which case you will need to use the Pearmail For SMTP mail object.
You should talk to your host about this error to determine what the best option is and whether they support Pearmail.