Spell Check in Mail - Checking only new content

I have a noob question regarding the Mail application. When I write an email, I have my preferences set to run spell check before I send it. However, the spell checker runs over the entire email including previous replies and messages I've received from other people. Is there a way to set it up so only my new message content (meaning what I just wrote) is checked? I looked around the Mail application, but could not find anything obvious. Any help would be much appreciated. Thanks in advance.

Sean