Why "r@.MISSING-HOST-NAME. or UNEXPECTED_DATA_AFTER_ADDRESS@.SYNTAX-ERROR."?

I already posted this question and thought it deserved a prompt reply: Why “r@.MISSING-HOST-NAME., UNEXPECTED_DATA_AFTER_ADDRESS@.SYNTAX-ERROR.” and sometimes “INVALID_ADDRESS@.SYNTAX-ERROR.” appear on the topmost part of the message window instead of the actual sender’s address? This prevents me from setting a rule for such addresses. This doesn’t happen with my other email clients.

Is nobody able to reply?