A free app for iPhone, by Spark for Mac.
Spark is a productivity tool that helps you stay focused and organized. It's a combination of an email client and a to-do list, but much more.
Spark is for professionals and their teams. Professionals use it to send and receive emails and collaborate with multiple people simultaneously. Teams use it to manage projects, send reminders, and keep everyone on the same page.
You can organize everything into cards, sort them into folders, and mark them as done when you're done. All the cards and folders are accessible on your desktop.
Spark makes it easy to collaborate on an email and share it with your team. You can create a discussion thread and have your teammates reply to you individually or in a group. You can also invite them to join the discussion and have them reply to you as well.
Spark has a real-time editor that makes it easy to write and compose emails. It's the best way to write an email that makes sense and is well-organized.