Sending a Mass Email is a two step process. The first step is to determine your mail list using Filters and/or Views.
Once you have selected your list, choose "Send Mass Email" from the dropdown menu just above the Types Column on your Lead List. NOTE: If you cannot see this option, it means your Account Administrator has not assigned you permission to send Mass Emails.
You can then compose a message on the editor, or load a message from your Message Library.
You can personalize the content of Mass Emails sent out of Open Leads by inserting tokens into the body of your message. This will merge the Lead Data from selected Fields in your Mail List into the message body.
Clicking on this option will open an email composition screen, where you can construct the email you wish to send. Once composed, click "Send Email" at the bottom of the screen and your email will be queued and sent to your list.