I presume you are using Gmail. Actually, both are in AllMail. That's its name. I do not recommend using the allmail folder in Thunderbird because it is a repository of everything and causes all your mail to be there twice. Other email services (to my knowledge) do not do this, so that is why I guessed Gmail.

Thanks for your response, but it doesn't address what is happening. I want to keep All Mail as back up and there must be a setting that sends my sent mail to the one folder. To further confuse matters - and I've raised this question before - some of my emails disappear altogether. ie I can't find them in In-box, sent mail, all mail or trash.

I am no expert on Gmail. All I can share is that Gmail is involved in your questions, not Thunderbird. Allmail folder is unique to Gmail and it is managed by Gmail.