It is very useful to auto-remove the old messages from your trash. By doing this, you do not have to regularly clear the trash of your Mail application. This feature can be enabled by following these easy steps:

1. Select Mail > Preferences... from the menu.
2. Click on the Accounts category.
3. Select the account whose Trash folder you want to configure.
4. Click on the Special Mailboxes tab.
5. Make sure "Move deleted messages to a separate folder" (for POP3 accounts) or "Move deleted messages to the Trash mailbox" option (for IMAP accounts) is selected.
6. Next, select One month/week/day old into the "Erase deleted messages when:" field.
7. Close the dialog and save the changes.

Now, your trashed mails will be automatically removed after they reach the selected age.