How to Turn On Spell Check in OS X or macOS Mail
To set your default spell-checking preference so that the spelling in every email is checked as you compose it:
Open the Mail app on your Mac. Click Mail in the menu bar and select Preferences from the drop-down menu. Click the Composing tab in the Mail preferences window. Next to Check Spelling, select as I type from the drop-down menu.
As you type your email, any misspelled words are highlighted so that you can correct them.
Alternate Method to Turn On Spell Check
To turn on spell-checking from within the composition window and avoid the preferences screen altogether:
Open the Mail app and a new email composition window. Select Edit from the menu bar. Click on Spelling and Grammar in the drop-down menu. Hover on Check Spelling. Select While Typing.
Spell Check in Older Versions of Mail
To check the spelling as you type in Mac OS X Mail 1, 2, and 3:
Open the Mail app. Select Edit > Spelling > Check Spelling As You Type from the Mac OS X Mail menu. If Check Spelling As You Type is not checked, click on it to add a check mark.
A Caveat With Spell-Checking
As in any program, spell checking is a matter of checking words against those in the program’s list of accepted words. If the word is in that list, it won’t be marked as incorrect or be corrected. In other words, the spell-checker can’t tell, for example, whether “to,” “two,” or “too” is correct in your sentence, so quickly checking over your email before you send it is always a good idea.
