Error: "454 4.7.5 Certificate validation failure, Reason: SubjectMismatch"

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We ran HCW >>> Exch 2016.

In a Hybrid Set up with O365.

 

Issue: Unable to send email from O365 to onprem.

 

- In the receive connector in the Edge onprem is the error; "454 4.7.5 Certificate validation failure, Reason: SubjectMismatch".

- Re ran HCW > No fix.

- Validated the Outbound connector from O365 to on-prem. It validated Ok.

- Test email was received by onprem user.

- Created a test user in the cloud with the initial domain (.onmicrosoft) and the user was able to send mail ok to onprem.

- Am suspecting the issue is with my custom domain.

- Because removing my custom domain from secure domains using set-transportconfig -TLSReceiveDomainSecureList $Null fixes the issue but this doesnot seem to be secure.

 

 

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