Exchange 2007 "reply" emails being bounced
This issue started happening around mid day yesterday.
Exchange 2007, outlook clients (2007 - 2013)
We can send and receive emails OK with one big exception; anyone outside our organization who replies to an email gets bounced with a "571: Delivery not authorized" message.
An external sender can send us a new email absolutely fine but if the same person replies to one of ours it's bounced.
Nothing (that I know of) has been changed at this end and until yesterday there were no problems.
I'm not coming up with any conclusive answers, does anyone have any ideas? I could do with a very quick fix here.