Have you tried simply restarting Thunderbird, and opening and closing the sub folders for the account?

There appears to be some instances where the list only updates after that happens.

Thank you very much to Matt for the help. In fact, the phantom folder disappeared soon after I sent my message. I think I must have accidentally done something like what Matt suggests. I think that re-starting Thunderbird (or closing & re-opening folders/subfolders) forces Thunderbird to re-synch with (in this case) Outlook. Maybe the overworked maintainers of Thunderbird could look into tightening this up, so that re-synching is a continuous process?

I have a vague feeling that Thunderbird and Office 365/Outlook don't always quite 'fit' with each other.

Thanks again to Matt for hi-speed reply -- much appreciated.

Roger Fern, Newcastle upon Tyne.

PS: Would it be good practice to shut Thunderbird down when you finish a session on the computer, and re-open it from scratch (re-load it) at your next session? I've been leaving my Mac on, and all the regular software (e.g. Thunderbird, Firefox, etc.) running.

Roger Fern, Newcastle upon Tyne.

The idea about closing and opening came from a bug report a read recently, so fixing, or tightening or doing something is on the cards. One of the issues with synced things is setting triggers, or in the parlance of Thunderbird developers, listeners to detect changes in the state of things like folders, names, existence etc. These can be quite expensive in terms of CPU or network, so you don't want to trigger even a request to see if the folder list has changed every 5ms, I did it once and crashed a corporate network through saturating the network. Everything is a trade off. But they are working on making the folder list more responsive in a number of ways and it is expected it will be entirely replaced with a new tree widget soon, so it is an area that is being looked at from a number of perspectives.

I would recommend a regular restart of all software. Even of the operating system. All software has issues, and one of the issues that occurs more frequently than is generally considered is memory leaks. This is where a particular program just uses a tiny (usually) bit more memory each day until the system is struggling to function. Thunderbird has had them, as has Firefox and under certain use cases probably still has them. Thunderbird is over a million lines of software code, some of which is best described as byzantine. Some remains essentially unchanged since Netscape open sourced their code for Netscape Suite.

Your operating system, whatever it is, is also prone to memory leaks. Generally developers are fairly conscientious in fixing reported memory leaks because low memory is when all kinds of software weirdness comes out to play. So everyone wants to fix such things when the cause is identified. But really no software is immune and the bigger the software project, the greater the risks associated.I leave my windows machines running for days at a time. Sometime a month when Microsoft forces a reboot for security and stability updates. Folk have been complaining for decades about the need to reboot after update, and Microsoft has done nothing through multiple major releases. My opinion is they do not fix the annoyance because they feel a restart is healthy on consumer devices.

Periodically my and my wife's phones just start doing weird stuff, a restart gets them ringing on incoming calls or whatever the issue is this time. One of those phones is Apple and one is Samsung, so I am not being provider biased when I say in my view everything needs a regular restart to stop those sorts of electronic gremlins. Turn it off for the weekend, or when you go to bed for the night. Or just restart it while you go out to lunch. It is cheap housekeeping than can save annoyance latter.