authentication required - rejected message

I have a lawyer that is trying to email me updates.  But he keeps getting sys.comcast.net rejected your message.  It goes on to state that "Authentication Required."

Now I have my lawyer's email address in my contacts and I have checked to allow 3rd party apps.

How can I get his email whitelisted, so that I can get his emails.  All other emails from all other "businesses" come through without problems.