Thunderbird has no fixed "delete after 30 days" policy. You can certainly set it to work like this, but equally well you can set it to never delete.

Tools|Account Settings|{select account}|Synchronization & Storage→Disc Space

If your Deleted folder is on an IMAP server then you also need to look to see what your email provider's deletion policy is. You are free to set Thunderbird to save its Deleted folder in Local Folders, safely out of reach of your email provider's server.

Tools|Account Settings|{select account}|Server Settings→Server Settings

Thanks, but dont work. On

Tools|Account Settings|{select account}|Synchronization & Storage→Disc Space I sync all messages, and I dont delete any messages.

Under Tools|Account Settings|{select account}|Server Settings→Server Settings I would like to save them on the server, not localy (in case something happens to my PC). So I have (freely translated) "When I delete a message: move it to trash folder: "trash on myaccount@gmail.com".

But when I delete the message, it disappears. I try to add an image here..

This isn't quite what you talked about in the first posting, where you led us to think the issue is messages deleting after 30 days…

Do you really have four different places for deleted messages?

1. folders that are called "deleted messages",
2. "trash",
3. and folders called … søppel (trash) and
4. slettede elementer (deleted messages).

I'd be worried about the ambiguity. Which one is really gmail's preferred Trash folder? And what are gmail's settings for aged message deletion?

Sorry if my english is confusing you. I dont know wich is preferred, and setting. How do I find out?

I think the one that gmail uses would be safest. I'd go to the gmail website and create a deleted message. Then in Thunderbird find which folder it uses by looking for that deleted message, and set that folder as the "deleted" folder for the account.