Mac Mail users can change the spell check settings using the option available under the Composing Preferences. Click the Apple icon -> Preferences and navigate to Composing Preferences tab.
The three options available for Check Spelling are as I type, when I click Send and never.
never – This option completely disables spell checker on Mail. This may not be a recommended choice unless you don’t see any problem sending emails with out doing a spell check
when i click send – This option would check for spelling and grammatical errors just before sending the email. This seems to be good except that when you are forwarding email it keeps checking for errors on the forward content as well. It will be night mare when it has chain of emails.
as I type
This is my recommended spell check option for Mac Mail as it runs the spell checker as and when you type. Right clicking on the highlighted error will display the list of suggestion and spell check option.