I have a client, we have recently moved to exchange 2013 with mail being proxied via a Sophos UTM. The system works well and the client recieves inexcess of 8000+ spam emails daily which the UTM catches.
One of the suppliers that emails in gets this message:
Re: Test Outbound
addressed to: firstname.lastname@example.org
has the following delivery status:
451 4.7.5 [internal] TLS negotiation failed
What should you do?
This message is an informational Delivery Status Notification and does not require any further action.
Delivery to the recipients indicated above has been delayed. You do not need to resend this message. The server will continue to attempt message delivery.
I do not see the attempted delivery on the UTM SMTP logs, where as all the other mail is there whether it is being delivered to the exchange server or being quarantined or blackholed. I have also excluded the domain.com from TLS checking on the UTM, still nothing.
It is my belief the issue is with the sending servers? Could someone please help.