I can ask thunderbird to cancel messages in Junk folder after a number of day I want to on my IMAP accounts. Why it is not possible for the Trash folder? I don't what to cancel them immediately, but I wait to have a option that cancel them after some number of days.
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You can right-click on any folder and change its Retention Policy. Eg set it to delete messages older than x days or keep up to y messages. You can set that account-wide or folder-specifically.
An option for days later might be useful, but as a practical matter, however, "private message: unique personal information" e-mails are a category that should be "quickly erased" whenever possible, so demand is likely to be low.
The program code should be as simple as possible, so I think this is a challenge in that balancing decision.