Channel meetings disappear

[Billy2Sips]

Good afternoon,

We have run into an issue whereby Channel meetings do not seem to work anymore.

We can create them, and they appear in Channel posts, but in the calendar they show as a faint entry with a loading circle and we are unable to interact with it, then it eventually disappears.

Nobody receives any invites and the only record of it is in the Channel posts.

We can create normal meetings outside a channel fine, and they show in our on-prem Exchange Outlook Calendar.

All our mailboxes are on our on-prem Exchange 2016 servers, we have Exchange Hybrid ticked in Ad Connect and all our users are synced to Azure AD.

I have found some pending and failed messages in Security & Compliance > Mail Flow > Message Trace

Defer - Reason: [{LED=450 4.4.317 Cannot connect to remote server [Message=SubjectMismatch] 

Fail - Reason: [{LED=550 5.4.317 Message expired, cannot connect to remote server};{MSG=SubjectMismatch}

In Exchange Admin Center our outbound connector from 365 > Our Org does not validate and also has a similar error:

Connect to 'exampleorg.com' from Office 365 - Failed - Connection Failed

450 4.4.317 Cannot connect to remote server [Message=SubjectMismatch]

But not sure if this is even required as all our mailboxes are on-prem, or if it's even related to our issue.

Any help would be very much appreciated.

Thanks

[#2 · Accepted solution]

We managed to solve this issue, posting our solution in case it helps anyone else with similar issues.

When running the HCW we chose our default front end server and certificate, but O365 was hitting our Edge server which uses its own self signed cert.

We changed the O365 outbound connector to use any digital certificate, including self signed.

We then added “include:spf.protection.outlook.com“ to our SPF record and after a short while everything was working as expected.