Assume you are testing out sending an email that has both an html body and a plaintext body. How do you verify that both bodies were sent correctly?
One way to verify is to view the email in Outlook Express. There is an option to view the complete MIME source of an email.
An email that has both HTML and plain-text bodies would contain a "multipart/alternative" part and underneath it you would find both "text/html" and "text/plain" sub-parts.