Many servers automatically empty the server Trash folder periodically, but it varies. If you have an IMAP mail account then this means your Trash will also get deleted because thunderbird imap accounts can only display what is currently on the server.

eg: gmail will auto empty the Trash and the Spam folder every 30 days. https://support.google.com/mail/answer/7401?co=GENIE.Platform%3DDesktop&hl=en

eg: yahoo auto empty Trash every 7 days and spam every 30 days https://help.yahoo.com/kb/trash-spam-folders-regularly-emptied-sln3518.html

eg:spectrum auto empties after 3 days Holds deleted email and automatically deletes permanently after three days. https://www.spectrum.net/support/internet/email-quick-reference-guide/

There is a setting in Thunderbird which can auto empty Trash/Deleted folder when you exit Thunderbird.

• Account Settings > Server Settings for account
• Bottom right under 'Message Storage'
• 'Empty deleted folder on exit'

So if you had this set up and exited Thunderbird Trash would get emptied.

You can set up a Retention Policy on the Trash/Deleted folder, but this is best done for POP accounts.

• Right-click on the Trash or Deleted folder and select 'Properties'
• Select the Retention Policy tab.
• set 'Delete messages more than N days'

Please note if imap account then the server policy will override.

Located info for bigpond / telstra We may delete any mail messages stored for 30 consecutive calendar days or more in the ‘Trash’ and 30 consecutive days or more in the ‘Junk’ folders of your Telstra Mail account. https://www.telstra.com.au/content/dam/tcom/personal/consumer-advice/pdf/consumer/bp-part-k.pdf