Follow these steps to setup your Microsoft Office 365 mailbox on your Android in the Gmail App.
Before you begin, update the Gmail app to the latest version:
Note: If the update option is not available, you're already on the latest version.
1. Open the Gmail app. Tap the menu icon in the upper left corner > Settings > Add account > Exchange and Microsoft 365.
2. Enter your full email address and tap Next.
3. Enter your password and tap Next.
4. You may see a Redirect request screen. Tap OK.
On the Incoming server settings screen, change your server to outlook.office365.com.
5. Follow any prompts you may get for security or device permissions, sync settings, and more.
You may also be asked to confirm Remote security administration and approve additional security measures. In this case, choose OK or Activate.
Your done!