If you use Outlook with Word as the editor and use Word's spell check as you type option, you've probably noticed the subject is not spellchecked as you type. In order to spell check the subject automatically, you need to enable the option to Check spelling before sending. (Tools, Options, Spelling). If you only need the subject line spell checked, you can cancel the dialog if its triggered by a word in the message body and the message will be sent.

Unfortunately, this setting is off by default (while check as you type is on by
default) so you need to remember to turn it on if you want the subject spell checked automatically.

Pressing F7 to initiate a spell check will also check the subject.