This tutorial tells you how to enable spell check before sending any email on Mac OS X Lion Mail App. The default behavior in Mail app is that the spellings will be check as we type the email.
Click the Mail icon and select Preferences from the menu list.
In the Composing Preferences, navigate to Composing section and select when I click Send for Check spelling drop down. This would enable spell check before sending email in Mail App on Mac OS X Lion.