Looks like lots of the apple users did not get the memo about login changes happening on June 15. This is a common question for the last few days. https://support.apple.com/en-us/HT204915 Read the section about 3rd party clients and app specific passwords.

Thanks, does this also affect the saving of drafts? Normally they get saved automatically from time to time as you write, but I get endless error messages to say that there was an error and they are not saved. I quite often get an error when I send an email. It goes, but I am told it was not saved in the "sent" folder.

I have not seen those messages so I can't help there. Maybe someone else will come along with an idea.

Ok, so I've solved the issue - by shear perseverance...

It is entirely due to the Apple Co introducing 2 factor authentication as someone above has guessed. To continue using Thunderbird on an Apple of any sort you need to do this - no option - if you want to access @me.com or @cloud.com accounts. The process takes an hour or two (took me three hours of guessing, reading instructions and asking Apple Community people). The Apple solutions pages on this subject are not good - they leave critical bits and pieces - "follow the screen directions" is not good enough!

Once this is done you can then generate "an app specific password" . It took me over half an hour to work out what you do with it! I discovered by accident that you have to close the app - Thunderbird and reopen it. This forces Thunderbird to ask for a new password on the offending account. It also has to be done for both incoming and outgoing emails - same password apparently.

Thanks for everyone's input.

Thanks for the followup details. i am sure it will help all the other apple people.

If you are satisfied with your issue, please mark it solved.