By default, Mail checks your spelling as you type. If this distracts you and causes you to lose focus on the message you are composing, you can set Mail to wait and check your spelling when you are finished.
1. Open Mail.
2. Go to Preferences.
3. Select the Composing tab.
4. Use the Check spelling dropdown and select when I click Send.
Now Mail will wait until you click Send to check the spelling in the message. Any misspelled words will be underlined in red.