Step 1: Sign into https://portal.office.com/
Step 2: Navigate to Outlook web app or access https://outlook.office365.com/
Step 3: Click on the settings gear icon on the top-right corener of the page and select View all Outlook settings.
Step 4: Switch to Compose and Reply tab and scroll down to the bottom.
Step 5: Click on Microsoft editor settings and turn off the Spelling and click OK.