[Good Idea] Ask if the email sender if they want to save the recipients in their contact list.
[Bad Idea] Prevent them from saving contact info if that user uses a competitor's free web based email
Here's an approximation of the display:
Your message has been sent to (the following recipients). Would you like to add them to your contacts list?:
email@example.com [can not add]
firstname.lastname@example.org [can not add]
email@example.com [Already a Hotmail Contact]