Office 365 (Outlook 2016 for Windows) - Check spelling before sending a message
You can check the spelling in your email message at any time by clicking Review > Spelling & Grammar.
To keep the Spelling & Grammar command always within reach, you can add it to the Quick Access Toolbar
- Click File > Options > Mail.
- Under Compose messages, check the Always check spelling before sending box and click OK.
Now when you click Send, Outlook checks spelling automatically. If there are no spelling mistakes, it sends the message right away. If you change your mind and want to edit the message some more, click Cancel to stop the spell check, and then click No.