You see only the messages started in Send—not your entire email inbox. But since Send is based on email, you can message anyone with an email address. No additional sign up steps and the people you message can respond from anywhere, even if they don’t have the app.
"Imagine you’re walking into a big presentation and someone asks you to find out if your colleague will be attending. Chances are, you don’t have your colleague’s phone number, but you will probably have their email address—especially if you’ve been emailing with them recently. You don’t have time to search your inbox, start a new thread, or even type out a subject line. You just want to ask that person, 'Will you be at the presentation?'"
The app works for people with Office 365 business and school email accounts, and Microsoft says it expects to make the app more broadly available in the coming months.
You can download Send from the App Store for free.