Without proper management, your Trash folder can get out of control and take up unnecessary space on your drive. You can either manually manage the items in your Trash folder, or you can set the interval that you want Mail.app to automatically remove these items.
1. Open Mail.app.
2. Go to Preferences.
3. Select Accounts.
4. Select the desired account from the left pane.
5. Select the Mailbox Behaviors tab.
6. Go down to the Trash section and us the Permanently erase deleted messages when dropdown to set the removal interval. Choices are Never, One day old, One week old, One month old, and Quitting Mail.