GMass - GMass allows you to easily schedule emails, send out mass emails and mail merge campaigns and more with Gmail! Click here to learn about more features!
Canary Mail - Privacy-aware email app for Mac & iOS
Campaign Genius - Send dynamic email content through any platform, instantly.
Polymail - Native email app for email productivity.
MailChimp Snap - Send simple, photo-based email campaigns from your phone.
HEY - Email at its best, new from Basecamp.