The built-in email tool in ExpressionEngine found under Tools > Communicate is sufficient for simple email sending, but it lacks many important features. FireMail is a complete email tool that replaces the Communicate tab, but that reuses the best of ExpressionEngine's built-in email libraries.
FireMail allows you to track opens, clicks and unsubscribes resulting from any email you send.
FireMail also adds a WYSIWYG editor when composing emails and supports batch sending.
Reviews taken from devot-ee