I noticed the request from an user with the internet message header which contained error:
firstname.lastname@example.org #550 4.4.7 QUEUE.Expired; message expired ##
I sent a test message and check message queues on EDGE server (Exchange 2010 SP2) . Related queue issued error:
451 4.4.0 Primary target IP address responed with:"451 5.7.3 Must issue a STARTTLS command first." Attempted failover to alternate host, but that did not succed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
Well I check also related send connector and it has used RequireTLS:$true.
CheckTLS.com gave me answer: TLS is not an option on this server – TLS Adv failed
Destination’s MX servers do not support TLS (STARTTLS extension). And all messages addressed there through the send connector which required TLS (encrypted transmittion) did not pass.
Solution? Maybe omit forced TLS, but is it secured? 🙂