is IMAP available with the mail provider? Are the accounts currently configured as IMAP?

Thank you Matt. No, I am on POP but I understand that my provider does offer the option of IMAP. I have also just heard elsewhere that changing to IMAP might help. I'll not go down that route just yet, though, as I understand that messages can only be viewed online and not automatically downloaded to the computers. I have, in the meantime, ensured that 'Leave messages on server' is checked on both computers, but the situation has not changed. I might just leave things for now, as my main computer is the one I normally use, and will receive messages. If I should need to use the other (standby) computer, new messages will arrive on that if Thunderbird is not turned on on the main computer. Thanks for your help, though.

well you have half the story with IMAP, it is synchronized, so Thunderbird default to downloading all mail. Leaving messages on the server, is not entirely satisfactory in my view (but I do it myself on a couple of accounts) any corruption of the popstate dat file for the account Or interruption to it being written and everything downloads again.

You might want to install this add-on "just in case" https://addons.mozilla.org/en-US/thunderbird/addon/remove-duplicate-messages/?src=ss Edit to add the link.

And can you please close this topic by clicking on one of the responses as the solution (your own are fine)

Thanks again, Matt. I'm leaning toward changing to IMAP and that add-on looks very helpful. I'll give it a try.

I have set up an IMAP account on my (new) computer and am pleased to see that it seems to work. The existing POP account remains.

The only drawback is that it is downloading ALL my old messages to my IMAP inbox, including those - perhaps the majority - which I earlier moved to my Junk folder!

Not an enormous problem, but I'd really like to ditch all the junk if at all possible.

unfortunately there are no real shortcuts, except if you have been marking junk as such there is a good chance the Bayesian filter will have learned enough to get most of them. I to have an IMAP and pop account of the same account in Thunderbird.... I left it to see if it caused problems, but I have seen none. Don't do an IMAP on your other machine until you have folders and mail as you like it on the first, then it will download correctly first time as they are synchronized.

Thanks Matt. Too late, I'm afraid! I now have two different in boxes on my computer, one for POP and one for IMAP. No real problem; just a slight inconvenience. Perhaps one day, when I am more conversant with procedures, I may be able to streamline things. I suppose that, having downloaded thousands of messages (mostly junk!) I do, at least, have a record on my computer of all messages received and sent should I one day decide to close my Virgin Media account.

I hope you don't mind me posting another question, but I now have POP and IMAP on Thunderbird. This results in my having to delete junk mail from both accounts every time I visit (as I get so much rubbish!).

Would there be any disadvantage of now removing the POP account and, if not, how do I remove it?

If your happy with IMAP it is truly time to cleanup.

Tools menu (alt+T) > account settings at the bottom of the account list in a hidden in plain sight account actions button. It offer to ability to remove an account.

Many thanks.