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

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.

Thanks for your efforts and I'm happy I was able to finally find this thread here!

I do have a small (but loyal) user base with Thunderbird, but I wasn't able to allocate much time to debug the efforts. Needless to say that I basically got somewhat "brushed off" by Microsoft first level support saying that Thunderbird wasn't a "supported client" and from that point on I had to drop the ball a bit (during a time I already had more than enough other important things to work on). So while I was able provide our users the workaround (showing all, not just subscribed folders) that all I could help them with.

I'll re-open a request and point to this forum thread. Yet I fear that Microsoft is simply not getting enough reports (I myself have ~ 200 teacher accounts, yet only 2-3 have reported such issues but when they are affected, the issue is perfectly reproducible (even with fresh Thunderbird installs). While I have > 2000 student accounts in the tenant I work with, not a single student has reported these issues either. It does seem to only affect a handfull of Thunderbird-using accounts (for now...).

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.

Thanks for sharing this. It seems we are at least starting to see consistent messages from Microsoft.

As we might be a little while before this gets fixed [...]

By our side, we have confirmation this won't be fixed in the near future/weeks.

Thanks to dozens of replies and messages to Microsoft Support, today we received this overwhelming mail (translated from Spanish).

I have asked internally and we are aware that there are several users affected by this problem,(Cannot subscribe to IMAP folders), however there is no solution available yet.Our engineers are trying to enable additional logging that might help them identify the cause of the problem,but this additional logging won't be enabled until sometime in January of next year.The only solution I can suggest for now is to use an email client that is not using IMAP.I'm afraid the only thing I can tell you is you have to wait until our engineers collect,analyze the logs and provide a solution. This will take time.I am very sorry that I cannot be of more assistance and that I cannot solve your problem right now.If you have any questions let me know, I remain at your disposal.

It was difficult and tedious to break that first layer of support, when they were constantly asking things not related to the problem, but at the end, with this answer we have a bit of relief since they mention they are looking into that specific bug.

So, moving things forward, we will stay attentive to the messages they may give us in the following weeks and hoping the fix won't come much later than Feb.