The Trash folder in Mac OS X Mail serves a number of different purposes. One of them is to act as a temporary holding area for messages you want to delete. To save space, you can set Mail to automatically "permanently clear" the trash after a period of time you specify.
A SIMPLE Trick to Clean Up Your Email Inbox – Apple Mail
From the Mail menu, select Mail > Preferences.
Go to the Accounts category.
Select the account with the Trash folder you want to configure.
