When the Windows user is not logged on then there is no Thunderbird running, and hence no messages can be downloaded.

Note, when just locking the screen, you're still logged on, and all programs you started continue to run.

I worded that badly.

I wish to distinguish between being logged in and the screen being locked & requiring the user to log in to get back to Thunderbird.

I believe programs can use [https://docs.microsoft.com/en-us/wind.../nf-winuser-systemparametersinfoa|SystemParametersInfoA] to see if the screen saver is running.

Is there a way to stop Thunderbird downloading email when the Windows user is not logged on?

So let's rephrase that to 'when the Windows user has locked the screen'.

I think the easiest would be to simply close Thunderbird before locking the screen.

Alternatively set up your account as IMAP so that you do always see the same set of messages on the server, regardless of which device you use to access your mail.

I do close Thunderbird mostly but sometimes I forget, especially if I wander away in a haze at 2 am. Note I usually put my machine to sleep too...but something seems to wake up the Windows 10 machine and TB starts downloading email. I haven't figured it out and it drives me nuts!

IMAP is not an option for me as I have very limited space with my ISP plan.