There is no built-in way to check spelling in Outlook on the web. 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 checking feature. For more information about how to check spelling in a particular browser, review the browser's help documentation.
If your web browser doesn't have a spell checking feature, you can search the internet to find out if one is 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 on the web, see our supported browsers.