Add comments to HTML or RTF email in Outlook 2007. 

The comments feature in Outlook basically works the same as it does in Word. When you add a comment, Outlook highlights the selected text, includes a balloon with your comment, along with your initial.

To add comments to a message:

Open the appropriate message.
Click Other Actions and click Edit Message.
Select the word or phrase to which you want to add a comment.
Press Ctrl+Alt+M and type in your comment.
Repeat step 4 to include additional comments.
When you reply to the message, after inserting your comments, they will be included in the message.