This is is caused by bad permissions in a receive connector. The fix:
- Open ADSIEdit
- Browse to Configuration, Services, Microsoft Exchange,
, Administrative Groups, Exchange Administrative Group, Servers, , Protocols, SMTP Receive Connectors - Open the properties for the receive connector(s) involved in the transmissions you are debugging
- Open the Security Tab. Under “Authenticated Users”, make sure “Accept any Sender” and “Accept Authoritative Domain Sender” are checked.
- Wait five or ten seconds, and try again.
- If still not, or if it works for a little while and then does the error again, you probably have severe issues in your Exchange. For a stopgap, you can set permissions for Everyone, but an Exchange rebuild is probably warranted.