This isn't a norm for Thunderbird, but may tie to your email account provider. Sometimes it helps to see which one is being used and then you can usually delete the other. Checking the settings for trash may let you override any defaults.

I'll ask my email provider, but I have 2 folders that contain deleted messages. The 'normal' Trash folder and then this Deleted Messages folder shows up from time to time even though I've deleted it and all its contents. I don't know how Thunderbird decides where to put the messages I mark as deleted. I suppose I should make a point of tracking what's going where. But my real issue is why this "deleted messages" folder exists at all.

Do check your settings for trash to verify that it is set to your preferred folder. You might also check the account online and delete a message there to see where it goes.

Yes, messages I delete are supposed to go to Trash. I've deleted the "Deleted Mail" folder for now, so there's nowhere else for them to go. Until Thunderbird decides to create that Deleted Messages folder again.

Just to repeat: check your account online. it may be your email provider's default trash assignment folder name. If IMAP, see if you can unsubscribe from it by rightclicking account and selecting 'subscribe'.

what do you mean by 'onine' -- I'm using my PC and go to the Thunderbird settings there. Most of my deleted messages go right to the Trash folder, which is how it's set. Every now and then, this new "Deleted Messages" folder appears. I don't understand why it shows up, when everything works the way it's set most of the time.

I meant online, via a web browser. That would let you see the account away from Thunderbird.