Error when attempting to synchronise Gmail

I keep receiving an error when attempting to synchronise Gmail. The error received is: “An error occurred. Cannot send message (None of the email recipients were accepted by the server.”) . Any idea as to the issue here?

Is it full text of error message you get? Can you please describe more what actions did you do before the error? Or maybe send some of the problematic mails that caused error to staron@emclient.com would help us.