Unable to write the email to the mailbox

I've just realised something important. If I click Inbox for whichever account is showing that message I see a different message:-

I found another question here from someone who'd had a similar problem and the reply was to rebuild the global database so I've started the process here but I'm wondering if anyone can confirm if that's the right approach for a problem like mine?

The inability to write to the mail file, which is essentially what that error is about, usually means some other process is using the file at the time it is to be written.

So the first thing to check is the Anti-Virus. What Anti-Virus are you using? Exit Thunderbird Access the Anti-virus and make your Thunderbird profile folder exempt from scanning.

If you downloaded Thunderbird from the official website: https://www.thunderbird.net/en-US/thunderbird/all/

Then, it's likely your Thunderbird folder to exempt will be here: C://Users/username/Appdata/Roaming/Thunderbird

If you exempt that folder then it will also exempt anything it contains.

Restart Thunderbird and report on results.

Thanks Todd, It's very early days but up to now, rebuilding the global database seems to have fixed it. I'll keep the Anti-Virus possibility in mind though, in case the problem returns. I'm just using the standard Windows Virus & Threat protection here (not sure what it's called these days but it used to be called Defender).