We encounter a strange issue on a SBS 2011 with Exchange 2010 SP3 server. We send emails through our ISP smar host. Everything worked great since a long time and a day this error appeared:
541 4.4.0 Primary target IP address responded with: "421 4.4.1 Connection timed out." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
We contacted the ISP and after some tests, they claim that the problem couldn't be from their side.
I then tried to create a brand new send connector with this smart host with DNS name and then with IP address with the same result. I created a NAT rule on our USG 100 to give the right IP from our pool to the server which had the gateway one and tested it successfully. The mails are coming from the right IP and the reverse DNS is OK too.
I tried successfully to send mail with telnet from our Exchange server through the ISP smart host server. The mail came with the good source IP and host name too.
What can I do now please?
Thank you in advance for your precious help, best regards,