Resolve Exchange Server 451 Error First check the port numbers 53 for DNS and 25 for SMTP protocol and also check the settings of Firewall. Open the Exchange Management Console and click on Edge Transport Server. Select the properties and go to internal DNS Lookups. Set the settings to default.
Hello, Nanis, the first thing I would ask you is about the Exchange CU version. Your problem can be solved by simply updating it to the latest version. Here you can find more useful tips on your case.
It is not so rare, so you can browse the Google and Microsoft community forum where users have the same problem.
What I can surely advise you is you need to backup your important o365 data. I've learned first-hand how important it isto have a clean copy of data to use for recovery. Even for O365 data.
