All credit to Petenetlive https://www.petenetlive.com/KB/Article/0001631 KB ID 0001631 Problem I upgraded my On-Premises Hybrid Exchange server recently, from Exchange 2016 to Exchange 2019. I remembered to add the new server onto the Office 365 send connector, but there was no mail flow between an on premises mailbox and an office365 mailbox? Solution This happens because, (even if you are using the same certificate on the new and old servers) the certificate that is used for TLS security between your on-premises Exchange server and Exchange online, does not get ’embedded’ properly on the send/receive connectors. You may see either (or both) of the following two problems. Check The Office 365 Mail Flow Log into Office 365 > Admin > Exchange Admin Center > Mail Flow > Connectors > Select the ‘Outbound’ connector > Validate this connector. Herein lies the problem! 450 4.4.317 Cannot connect to remote server [Message=SubjectMismatch] [LastAttemptedServerName={ on-p
