Google may have just given Gmail a full-scale upgrade, but Microsoft is on its tail, at least for business users.
In the Windows client, the biggest upgrades are to the calendar. It will now remind you in advance when you need to pay bills, and when you're booking a meeting, it will auto-suggest available conference rooms or other locations where you frequently have meetings. If the meeting is off-site, you'll get a notification when it's time to leave.
On iOS and Android, you'll finally be able to sync your draft folder with the desktop client. That means you can start an email on one device and finish it on another.
The Android app is getting Office Lens, which is great for meetings where you write on whiteboards. The app will clean up the image and crop it to make it look cleaner and more professional than something that was just written on a wall. Quick Reply, a box at the bottom of the email to start replying immediately, is also coming to Android, but is already available for iOS.
There's a whole bunch of business-specific features, including RSVP tracking for meetings and the ability to block external content to avoid tracking on mobile coming as well. You can read Microsoft's full documentation here.