Annotating email from someone else

I use Outlook 2000 on various computers. On one machine, I can
annotate emails that I received with notes that remind me what points
I addressed, as well as elaborating on how. (Color-coded annotations
so as not to confuse them with the original text). On a different
machine, I am unable to modify received messages. Is there a setting
that controls whether not I can modify incoming email by adding
annotations?