Right-click on your Deleted folder > Properties > Retention Policy and verify whether there is a setting to delete messages after one week. Another possibility: if your account is IMAP and if you use the same IMAP account on a smartphone email client there might be a setting to delete messages from Trash after a week. Or your email provider has a setting to delete Trash messages after a week. Verify on web mail.

Thanks for the hints. In the deleted folder was on "Don't delete any messages" and "Always keep starred messages". This last point I turned into "off" now. On the smartphone was no setting to delete messages. At GMX neither.

Frankly I don't know the answer. Still one idea: go to Account Settings > account name > Server Settings > Message Storage and verify that the box "Empty Deleted folder on Exit" is unchecked.

Thank you so much. The box you told me was unchecked. In the Server settings at "When I delete a message move it to this folder" bevore was signed "Deleted", I changed it now to "Gelöscht" - and now the item "Deleted" is no more shown up, strange.

Let's see what is going on now.

Have a nice week, thank you

I have never seen a folder "Gelöscht" in a German Thunderbird version. By default the German term for "Trash" or "Deleted" is Papierkorb.

Before in this account I had "Papierkorb" and two times "Gelöscht", now there is only onte time "Gelöscht" in the drop-down-menu and "Papierkorb" is vanished. In another accout there is "Papierkorb" and "Gelöscht", in a third account only "Papierkorb". In this third accout are still the mails from the 4th of february until today. Where I did the change from "Papierkorb" to "Gelöscht" are left only the ones of today.

I throw in the towel, I can't solve your problem!

In spite of all, thank you very much! I learned now, that it's not a simple problem. Maybe I have to deinstall the program.

Now I had the idea to ask the provider GMX to this problem. He tells me , that in the folder "deleted" all mails are deleted after 7 days to save space! But of course they didn't tell that anywhere when changing. So i have to put every mail into the folder "archives".

Who told you that Trash/Deleted/Deleted Messages/Deleted Items were folders to store messages definitely ?

The messages I want to store I put into the folder Archives. But sometimes I realize only later that it would have been better to store an email. In this case I was happy still finding it in the folder Deleted. So I got used to use this folder like an insurance in the case of needing a (almost) lost email. And then from one day to another this insurance was destroyed...