Have a client with a formerly working fine Exchange 2013 server. Monday it decided to start refusing "some" mail. Nobody (to my knowledge) changed anything.
1) Mail is rejected with the "571 Delivery not authorized, message refused" message.
2) Server does accept some mail from the internet.
3) I can send original messages to them and receive messages from them, but cannot reply to a message sent to my domain. Get the above message.
4) manual smtp sessions via telnet seem to work.
5) only error I can find in the Frontend log file is sessions that fail end with a "Remote(socketerror).
I disabled the maiware processing via MS script.
any idea appreciated,