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O365 hybrid connector to onprem failing TLS
We're having issue with the connector to on-prem from Exchange Online If we enable the TLS it fails with the error Cannot connect to remote server [Message=451 5.7.3 STARTTLS is required to send ma...
Found that the STARTLS command availability depends on the security options combination enabled on the connector.
created a connector with just TLS enabled and worked
What is the output of Get-ExchangeCertificate on your on-premises server instance(s)? Take a look at:
https://learn.microsoft.com/en-us/powershell/module/exchangepowershell/enable-exchangecertificate?view=exchange-ps
and make sure you enabled the correct valid cert for smtp.