Office 365 (Outlook on the web) - Check spelling before sending a message
Some email programs, such as Microsoft Outlook, include a spelling checker. Outlook Web App relies on your web browser for checking spelling. That's why you don't see a spell check button or spelling settings in Outlook Web App.
Spell checking is available in Internet Explorer 10 and later versions, and current versions of Firefox, Chrome, and Safari. Check the options for your web browser to learn more about how to use it to check spelling.
To learn more about the browsers that are compatible with Outlook on the web, see Supported browsers for Outlook on the web