Recently we had a drop in server power and after everything came back online, we picked up that mail wasn’t delivering from a hub. Event ID 1035 was logged in the application log and a every long error message in queue viewer logged: 451 4.4.0 Primary Target IP Address responded with: “235 0000083CY…….”
Restarting the Transport service on that HUB didn’t fix the issue. This hub server was part of a smart connector, rebooting the other source servers part of that connector resolved the issue.
Hope it helps.