I don't think we need to display them at all. gmail has a good solution for it: "there are 4 deleted mails in this thread >show them ->delete them forever"

Makes sense. I would suggest to:

• Hide all trash mails completely
• Have a "Show delete mail" button (with the wording you suggested), which unhides the mails that can then be individually treated (meaning we still need a way to indicate that a mail is actually already in the trash folder).