As anyone whose used Outlook for years knows, setting Outlook to mark quoted text with a prefix, such as a ">", results in spell check checking the entire message.
Using Word as your email editor and HTML as the default message format allows you to use a prefix and not spell check the quoted text. (Outlook 2002 and 2003)
To change your quoting style, open Tools, Options, Preferences, E-mail Options.
Published January 20, 2004. Last updated on May 16, 2011.