do you have a VPN? They are notorious for breaking email.

Thank you for your message Matt. No, I don't have a VPN. Do you know of any other things that I could investigate, please?

Check webmail for a corrupted message:

https://support.mozilla.org/en-US/questions/1301414

If that doesn't help, check the server settings and possible interference from AV apps:

https://support.mozilla.org/en-US/questions/1334821

Thanks for your reply. Being a bit slow at these things, could you just help me with some of the wording in the first link? It says " Access the account in webmail, empty the Bulk and Trash folders, and delete Inbox messages one-at-a-time, starting from when the problem began, until normal downloading to TB resumes." Does it mean "Open your webmail account and do this stuff"? Maybe it means "Access your TB account via webmail". How would you do this?

Sorry but a little more help here would be great.

Whatever account you have in TB, it can also be viewed by logging in at the mail provider's webmail site, e.g. gmail.com, or your ISP if the account is with them. Look for the messages that arrived in the webmail Inbox and also in TB, and then the messages that arrived afterwards in webmail but not in TB. One of the latter set, perhaps the oldest one, is possibly blocking the ones that came later. Delete it, then empty Trash, and see if messages start arriving in TB.

Thank you for your reply. Is there another way to track down the corrupted email? Unfortunately, since downloading the TB app I have received no email so I can't follow through with the strategy you outlined.

The method only requires that you delete messages from PlusNet webmail and watching for downloads to TB to resume as normal. If deleting messages fails to uncover the corrupted message blocking the others, the problem may be due to other factors as suggested in my first reply.

I've tried to start this task but realized soon that I don't want to delete some of my webmail. Does archiving it, such as I think that you can in MS Outlook have the same effect as deleting it?

Instead of deleting them, move them to another folder; archiving is basically the same as moving to an Archive folder.

OK, thanks again for the reply. I have archived most of the emails from August using my desktop MS Outlook app. I've had no download of email to my TB app yet but I'll keep going and archive July's email.

...sorry, my misunderstanding again. Do mean a folder stored on my PC's hard drive is needed? As these would be .pst files, I understood that they could only be opened in MS Outlook? Alternatively, did you mean that moving them to an archive within MS Outlook, would achieve the objective of this exercise?

Thanks for any update.

I meant access the account in webmail, create a folder, and move the messages to the folder instead of deleting them. Then see if downloading of new messages to TB resumes.

If you have the account set up as POP in multiple mail clients, downloading with one of them will delete them from the server if so configured (usually this is the default), which means there won't be any messages to download if another client connects afterwards.

Thanks for your message. I've moved all of my webmail from June, July, and August to an archive file in webmail but unfortunately, no breakthrough with regards to a Thunderbird download. My account is an IMAP one so the POP issue won't occur.

Where next do you advise? Archiving more webmail or turning to your advice on AV apps. I've checked the settings for servers and ports against my ISP's recommendations. I only have Windows Defender on my laptop so can I assume that this should not interfere with TB?