You can enable the spelling check any time you want during message composition by clicking “Check Spelling” icon in the message composition dialog.

check spelling

Then the “Check Spelling” should be started and the similar screen should be shown.

check spelling correction

The spell checking dictionary is for English. And the mis-spelled words will be highlighted in red. Left click the words will give a list of correction candidates. You can left click one of the best correction candidates to finish correction of this word. You can resume typing the mail contents at anytime by clicking “Resume Editing”.