Symptom: Mail to address(es) at a client domain is returned with the following error:
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
(generated from firstname.lastname@example.org)
Cause: Improper setting of the default email address via the control panel.
Resolution: Log in to the control panel, select the mail management option, and reset the default email address. For any address other than the master username, the default address must be a full email address in the format email@example.com.