If you use Outlook 2007, you can add comments to an HTML or RTF email, using the same Reviewer comment used with Word. (It doesn’t work with plain text.)

You need to be in Edit mode (Other Actions button, Edit message) then select a word or phrase and Ctrl+Alt+M to insert a comment.

If you reply to the message after inserting the comment, it will be sent along with the message, unless you delete it from the reply. Like this[DP1]

I haven’t decided how useful this is. A lot of users want to add comments to messages, but they usually ask for a sticky note ability and don’t want to edit the message. Using the For Follow Up field puts the comment in the Infobar, where It’s easy to see, but its limited to 100 characters.

[DP1]This is a comment.