I have a simple cpanel-administered www hosting account (wherebarney.com), and for well over a year I've used Google Apps for your Domain to handle the email, using gmail.
Recently the majority of (but not all) inbound mail has been bounced back to the sender with 'Please turn on SMTP Authentication in your mail client'.
I can find lots of advice about how to secure my domain to stop spoofing etc, but AFAICT this only affects outbound mail and spam, not inbound mail.
What else should I look at?
(reason: 550-Please turn on SMTP Authentication in your mail client. )
----- Transcript of session follows -----
... while talking to wheresbarney.com.:
DATA
<<< 550-Please turn on SMTP Authentication in your mail client.
<<< 550-mx0a-00082601.pphosted.com [67.231.145.42]:48701 is not permitted to relay
<<< 550 through this server without authentication.
550 5.1.1 <barn@wheresbarney.com>... User unknown
<<< 503-All RCPT commands were rejected with this error:
<<< 503-Please turn on SMTP Authentication in your mail client.
<<< 503-mx0a-00082601.pphosted.com [67.231.145.42]:48701 is not permitted to relay
<<< 503-through this server without authentication.
<<< 503 Valid RCPT command must precede DATA