MassMailer - MassMailer lets users send mass emails from within Salesforce.
Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
InstAddr - Disposable email address application.
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
Greatmail - Greatmail is an email hosting service provider built for businesses of all sizes.
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.