Problems with uncontrolled Trashing

I have some problems with Thunderbird regarding the content of the Trash folder/box. Again and again, I experience that a lot of the mails in the Inbox are gone, and then I find them in the Trash. Even though I have NOT trashed them, and in the Trash there are also e-mails from other folders, that I have not Trashed. So it seems the e-mail program are trashing e-mails randomly, out of control. Whats this ? Anythnig I should check or do ?
Martin