The built-in email system is designed for emailing active members of your site whom have requested to receive emails from your website. A good example of a good use for this tool would be to send a monthly newsletter to the users that have signed up for Newsletters: 

Subscribing to the Newsletter -

The subscriber will fill out the pop-up module:

After clicking on "Subscribe Now" a success alert will appear:

Where to Find Newsletter Subscribers -

Additional Information:

>> View Contacts Folder

How to Send Newsletters -

Additional Information:

>> Emails » Send Newsletter » New Email Newsletter

>> How To Create A Newsletter Template

>> Unsubscribing From The Newsletter

Editing the Newsletter Design -

Additional Information:

>> Footer And Newsletter Design

Unsubscribing from the Newsletter -

>> Unsubscribing from the Newsletter