This tutorial talks about how to enable the automatic deletion of emails in Google Apps. The default settings for email retention is set to “Do not delete email messages automatically”, but you can change this using the options available as part of Email Settings.
Login to your Google Apps account as administrator and click the Settings tab on the Control Panel. Navigate to Email settings page and scroll down to Email Retention section.
Select radio option with label as Automatically delete email messages older than and specify the number of days after which you want the mails to be deleted. Google Apps also provides option to specify whether you want to Move messages to the Trash folder or Permanently delete messages.
After making the necessary changes, click the Save changes button.