There is no built-in spell checker available in Outlook.com. Here are other ways you can check spelling:
Use your web browser to check your spelling. Most web browsers such as Microsoft Edge, Internet Explorer 10 (and later), Chrome, Safari, and Firefox have a spell checker feature available. For more information on how to check spelling in a particular browser, review the browser's help documentation.
If your web browser doesn't have a spellchecker, you can search the internet to check if there is a spellchecker add-in available for your browser.
If you're using Windows 10, select Start > Settings > Devices > Typing, and then search for Autocorrect misspelled words and Highlight misspelled words. Turn on both settings.
To learn more about the browsers that are compatible with Outlook.com, see our supported browsers.