If you receive and delete a lot of emails, trash can be filled with the emails you no longer need.
See also: How to access activity manager in Thunderbird You can empty the trash by deleting emails one by one.
If you have a lot of emails in the trash, you can delete them all at once.
If you want to delete one email, open Thunderbird and click Trash on the left side.
If you have several emails in the Trash and you want to delete them all at once, right-click on the Trash, and click Empty Trash.