RM571310 - Outlook: Spell check before sending emails in new Outlook
Microsoft 365 Roadmap
Summary
If you don’t want to check spelling manually for every message—or you’re worried you might forget!—you can set Outlook to check spelling for you every time before sending emails.
If you don’t want to check spelling manually for every message—or you’re worried you might forget!—you can set Outlook to check spelling for you every time before sending emails.