Newton is a fully featured mail client available for Android, iOS, macOS and Windows. Works for watch too. It's based on cloud so you can connect it to any account and get fast search, push notifications, read receipts (PREMIUM), etc
Disc works in your device.
Scaledrone is a realtime messaging service and platform.
Send live updates, create chatrooms and collaborative tools.
Most advanced marketing automation platform with multi-channel support via email, SMS, in-app and push notifications. Built-in machine learning and AI to automatically track subscriber events and segment them for real-time and relevant communication.