Please try the following as it helped me to resolve an issue that sounds exactly like yours.

Use this link: https://login.yahoo.com/

reset the password to the same password you are currently using to access webmail and which is being used in your email client.

If for some reason, you are redirected to anything other than exactly what that link says above, then clear your browser cache and re try.

BT has a habit of redirecting you to a different link to logon, but you must make sure you use the one that only says https://login.yahoo.com/

If this works then it means BT and yahoo servers were out of synch.

Thank you for your suggestion. Very plausuble. However, before I received your response and had a chance to try it, I had a second on-line 'Chat' with BT. This time I pointed out to them that I also had the same problem with my mobile phone, so it was NOT a Thunderbird issue alone. They promptly raised my issue to a "level 2 responder" who was going to call be back this evening. Surprise surprise, within 20 or so minutes, my held up mail all came though to both my phone and to Thunderbird. And its all been fine since. Un-surprisingly, BT have not yet got back to me. What do you make of that? Might it be the synch issue you raised, that BT sorted from their end rather than me doing it from my end?

In the early days of TV, it was not unusual for regular tv transmissions to be replaced by a screen saying "do not adjust your set". The intention of this was to advise the audience that there was a problem, usually at the originating studio, broadcaster or transmitter, and nothing could be achieved by users attempting to re-tune their TV sets.

I think you had one of these situations, where the best policy is to wait patiently because there is very little you can do about it for yourself, except perhaps watch a different channel. You may have been lucky in getting the attention of a competent technician who was able to escalate the issue and have the servers re-set or whatever.

Personally I wouldn't want to have to rely on bt's email service. It has always been subcontracted out and has never been the most reliable system. The idea that there are two or more servers that need to be synchronized tells me that their system is more complicated and fragile than it need be.