What's happened to my Thunderbird?

It has a solid black square inside a hollow square. What does that mean?

Anyway, I haven't done anything to make it happen. Some file must have got corrupted. I can still send mail, and it's saved in the Sent folder. It just won't write incoming mail to the mailbox.

"indeterminate"

It's definitely not full. I don't think the error message can be true; I think it must be that a file has become corrupted. I just can't work out what's the matter. The error just came out of the blue, about the same time as it started asking me to re-enter the passwords every time.

I need a way to reset thing back to how they should be, without losing data.

Well I have in the past copied my entire email data directory without issue.

In fact it resides on a NAS and if the NAS isn't up the email still sort of works.

Because the configurations for thunderbird are (can be?) held in a different file system to the data, So it is entirely possible to move the data somewhere safe, and delete the whole configuration for Thunderbird and recreate it.

Setting where the files and folders reside is a global configuration part of Thunderbird on a per account basis

If you can right click on your main email name under which all your mail is stored. That shows you where it thinks its trying to store stuff Make sure it IS where you want it to store stuff.

Try moving all your folders somewhere else and point the mail at that location.

FWIW I've fixed these problems by restoring the Thunderbird data from a backup dated 28 August. I haven't lost any data as I leave (recent) email on the server.

It would be nice if applications checked the validity of any data which could affect the operation of the program before using it. Most data doesn't, some does.

A while ago, an update to the *data* of a security update crashed a vast number of Windows computers worldwide:

"Let’s just step back and consider. A single software update by one company collapsed half the internet."