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: 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:
Spell Check in Older Versions of Mail
To check the spelling as you type in Mac OS X Mail 1, 2, and 3:
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.
