SMTP Relay. Odd behaviour. SMTP gurus please help

I have problem with smtp relay errors where mail to one user name is ok to another fails in the same domain.

Mail received for abc.def@domain.com is rejected: “Attempt to relay mail to abc.def@domain.com rejected for policy reasons. Relays to recipient’s domain denied in your configuration.”

Mail to abddef@domain.com is accepted and delivered.

both addresses appear in the person document and are available in the $users view.

This is consistent at least. Mail to any address with a “.” in the middle of the name (abc.def) is rejected but with out the “.” is accepted.

What is wrong?. How can one address to a domain be accepted but another rejected.

Thanks in advance.