📌1. About 2 weeks ago several customers complained that federation trust doesn’t work in their Exchange environment. Federation Trust was used to share free \ busy information with their partners (federation trust + organization relationship).

📌2. In Exchange Server application Log in target organization we could saw specific error:

The token received by the federation partner does not contain a proof key. Check the application event log for more information.

📌3. Test-FederationTrust doesn’t work with specific error below:

Id      : TokenValidation

Type    : Error

Message : Failed to validate delegation token.

📌4. HTTP proxy logs on source Exchange Server contains errors 500 related to cross forest autodiscover

ASAutoDiscover/CrossForest/EmailDomain ... WebExceptionStatus=ProtocolError;ResponseStatusCode=500

We investigated the issue and wasn’t able to find any issues related to Exchange configuration. We tried to recreate federation trust and renew federation certificate, but it didn’t help.

After a couple of days we’ve got information, that similar cases (mostly related to federation issues in hybrid environment) were opened in Microsoft support. I don’t have any information on the root cause of the issue, but soon this issue was resolved for cases that we had.

💡If you experience described issue that would be a good choice to open a support case with Microsoft, as looks like issue is MFG related.

End.