Unable to subscribe to Office 365 IMAP folders from MUA since 2022-11-01

An individual in our tenant reported the symptoms of this problem to me, but it took some time before I could verify the problem, since none of my testing accounts manifest it. Then I found this thread, which has been helpful in establishing that our tenant is affected. (I reluctantly connected my own account to Thunderbird with IMAP.)

I have reported the problem to Microsoft through official channels, and they have confirmed that it is a bug. They stated today that there are changes coming to logging on their side that should help them identify the cause, but also that these improvements will not come until January. Here's to hoping!

Good news is this morning we found out that the issue got solved without notice.

We received a message a week ago about Microsoft Support saying they would start to see progress in around 15 days, but the solution came much faster.

Haven't received yet a confirmation email by them, but the folders seem to work fine in all the users affected and in all tenants.

A handful of affected people per tenant as you are seeing, appears reasonably typical - and that is why service desks and SAs might genuinely think it's a user/local issue, and probably why Microsoft tried to claim that in the early days.

However, I think we are now well past the point where Microsoft have had to admit there is server-side fault/bug/mis-configuration - and IMO, we reached that point on the 23rd November when their own test script issued to me exhibited the same issues as the various affected MUAs. But Microsoft have still refused to declare the incident openly on Service Health (it deserves at least an ADVISORY given it is clearly affecting customers right across Europe), and there is certainly evidence of 1st line Microsoft people pushing back on reports after that date which is completely unacceptable given the amount of evidence in this thread.

Regardless of whether Microsoft have finally acknowledged this in various updates to individuals, my advice to SAs still stands that you should raise your own ticket.

Yes, that is almost word for word the same response that I received. I want to add two other pieces of information:

1. I urged Microsoft Support to add the issue to the Service Status section of the M365 Admin Center. The fellow replied that he forwarded my request to the team responsible for those notices. We’ll see.
2. The test accounts in my tenant that had still worked with IMAP appear not to work anymore. But I need to confirm that my last attempt wasn’t a fluke.

I've just chased Microsoft on the back of my support ticket, but I'm not expecting a rapid resolution to it, given there is a workaround available to display all folders.

Remember too that the number of users and O365 tenants affected is going to be a small percentage of the overall O365 service - simply because most corporate users of O365 will have users on Outlook accessing with the standard Microsoft connection protocol MAPI, and not IMAP. None of my Outlook users are affected by this issue for that very reason - and it was Thunderbird using IMAP where the problem first surfaced for me.

All I can suggest is those of you who manage O365 tenants open your own support ticket in Admin Centre so Microsoft can at least see who is affected. By all means cross-refer to mine which is ticket ref. 33661676

If you are an end user and certain you are seeing this exact same issue, then maybe contact your service desk/O365 sys admin and ask them to do the same.