This can happen if the popstate.dat file becomes corrupt. The download should reset it.

Thanks but I don't understand what you mean by the download should reset it?

Thunderbird updates the file at shutdown with the number of last downloaded file. If, on restarting thunderbird, the downloads repeat, that is an indication that something (an antivirus? VPN?) is preventing the file from being saved.

But how do I fix it? When you said the download will fix it do you mean that I have to wait for all the old emails to download?

My problem is that thousands of emails are downloading but not the latest emails. So, I'm not getting any of the emails from today or yesterday.

I do not know. I shared what information I knew. That one file is the linchpin for POP. All I can suggest is to periodically do a backup of the popstate.dat file. Others here may have POP expertise, but I do not. This article may assist you. https://kb.mozillazine.org/Popstate.dat

I have backups of the popstate.dat file but I'm not sure which to restore. For example, I have two yahoo accounts but I have 8 pop.mail.yahoo.co.uk folders that go up to pop.mail.yahoo.co-7.uk.

Each has a popstate.dat file. I have no idea why I have so many folders or which files to restore.

What has really confused me is that I have logged into my email account via the web and deleted a whole load of emails. But TB is somehow still downloading them. How when I have deleted them?

I am no guru regarding Yahoo and avoid when possible. But, there should be two account folders, not eight. Go to 'accout settings' pane, click 'server settings' pane for one of the accounts and then look in lower right at local directory. Do that for each account. That should confirm the account folders that are in use. As far as having deleted messages while online, all I can say is that they obviously were not deleted and that may be a question to Yahoo. a