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.

First I want to thank everyone who has collaborated on pinning down this issue. This bug is stalling our organization's migration from G Suite to Office 365, since forcing every user onto the Outlook desktop app is not an option for us.

I have opened a support ticket with Microsoft pointing to this thread, requesting to be notified when the bug has been fixed.

1. Am I correct that there is currently no known work-around to this bug?
2. Is this bug acknowledged by MS and listed on some public bug tracker or similar where progress may be monitored?

Hello! To your questions:

1. The fundamental 'workaround' is to use Outlook for the Web. No client but a browser is needed for that. If you must use an IMAP client, e.g. Thunderbird, then disabling the option that shows only subscribed folders is a workaround.
2. Yes, the bug is acknowledged by Microsoft, but not publicly, i.e. it has not yet been listed in the Service Status section of the Microsoft 365 Admin Center.

First I want to thank everyone who has collaborated on pinning down this issue. This bug is stalling our organization's migration from G Suite to Office 365, since forcing every user onto the Outlook desktop app is not an option for us.

I have opened a support ticket with Microsoft pointing to this thread, requesting to be notified when the bug has been fixed.

1. Am I correct that there is currently no known work-around to this bug?
2. Is this bug acknowledged by MS and listed on some public bug tracker or similar where progress may be monitored?

From what we collectively know about the issue, it's highly unlikely it will affect "every user" you migrate. It will likely just be a small percentage of them - but there is no way of knowing in advance what percentage and which ones as we don't know what is actually causing it yet (and neither it seems, do Microsoft).

As David L says, there is a relatively simple (one tick-box) workaround for affected Thunderbird users, but if you plan to use some other IMAP mail client, you'd need to look at whether there is equivalent functionality in the settings of that product to override IMAP Subscription, which is the bit that is broken for affected users.

Thank you for mentioning the Thunderbird workaround!

I was contacted by Microsoft support who claimed that IMAP is no longer supported since Jan 1 2023. I cannot find any mention of it online, perhaps the representative was refering to IMAP Basic Auth being deprecated. https://techcommunity.microsoft.com/t5/exchange-team-blog/basic-authentication-deprecation-in-exchange-online-september/ba-p/3609437