Thunderbird IMAP responded: User is authenticated but not connected

Thank you for your answer. I get the following error message in german:

Die Nachricht wurde gesendet, aber es wurde keine Kopie im Gesendet-Ordner (Sent Items) gespeichert, da es zu Problemen bei dem Zugriff auf Netzwerk oder Dateien kam.Sie können es erneut versuchen oder die Nachricht lokal unter Lokale Ordner/Sent *** Email address is removed for privacy *** speichern.

The error means that Thunderbird cannot save a copy of a sent mail message and is giving you the option to save a copy of the message to a local folder. I have seen that error message before and it can have several causes. This error might be unrelated to the IPv6 error.

Der Fehler bedeutet, dass Thunderbird keine Kopie einer gesendeten E-Mail-Nachricht speichern kann und Ihnen die Möglichkeit gibt, eine Kopie der Nachricht in einem lokalen Ordner zu speichern. Ich habe diese Fehlermeldung schon einmal gesehen und sie kann mehrere Ursachen haben. Dieser Fehler hat möglicherweise nichts mit dem IPv6-Fehler zu tun.

https://support.mozilla.org/en-US/questions/1299140

Try to repair your sent email folder:

Versuchen Sie, Ihren gesendeten E-Mail-Ordner zu reparieren:

1. Klicken Sie mit der rechten Maustaste auf den Ordner für gesendete E-Mails (right click on SENT email folder)
2. Eigenschaften (properties)
3. Allgemein (general)
4. Reparieren (Repair)

While I'm glad it seems the fix is rolled out now, we're now almost 5 business days later after I've opened my support case, still no update at all. (?!)
What's the experience from others who have had cases opened with Microsoft?

While I'm glad it seems the fix is rolled out now, we're now almost 5 business days later after I've opened my support case, still no update at all. (?!)
What's the experience from others who have had cases opened with Microsoft?

Like watching floorboards warp really. Trouble is, I know logging it has to be done.

I had a reply yesterday from the first line engineer asking for details of the Thunderbird config of one of my affected users. I pointed out that it was affecting many tenants, many MUA products, appears to have hit us all at the same point in time, didn't involve users changing config, should not have required the type of fix we have all had to do, and might even have mysteriously fixed itself (can't confirm on my tenant yet). It will be interesting to see what the next update says.

I think the harsh reality is Microsoft treat the IMAP interface as if they are doing us some great big favour by providing it. They offer no SLA on availability of it that I can see, and they can always argue (and probably would!) that a failure can never be business critical because users can always log in via browser and Outlook Online. Messing up IMAP won't impact their corporate users on MS Outlook either as they won't be using the IMAP protocol. Finally, people using a third party MUA product gives Microsoft the perfect excuse to deny all knowledge and blame the MUA/user/config until the fault is actually PROVEN onto them (as is the case with IMAP subscription they broke for some users back in November and still haven't fixed over two months later).

Add this to the fact the user base for Exchange Online is probably hundreds of thousands of tenants and millions of end users across Europe alone, and this thread with a few hundred people reporting the same issue will barely register on the scale I'm afraid.

In defense of Microsoft's first line, I actually find I get a reply quickly when I raise a ticket (typically within an hour) - but if it needs to go to Product Group as these sorts of complex/indeterminate issues do, THEY don't even seem to be accountable to their own first line, let alone us. They don't seem to feel obliged to put widespread incidents onto Service Health either - yet I see some trifling issues make it onto there.