If you are receiving this error, lets try to remember……
Probably, you had a hybrid Exchange with Office 365, right?
Did you remember, if you have changed any external certificate?
If yes…. lets check the connectors…..
From: Your organization’ email server to: Office 365….
The config is setted to verifying that the subject name on the certificate?
The certificate was changed and you forgotten to change it in the Office 365 portal?
So, you can solve it quickly, change the connector to accept the emails from IP instead of Certificate……
Just, select By verifying that the IP address of the sender server matches one of these IP addresses that belong to your organization, click + and add the IP’s…..
After it, go ahead and complete the next steps……
Now, you can check the queue in Exchange Server and send a email to test the solution…..
It is a simple post, but i feel that will help a lot of us….