[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?:

babixl@yahoo.xom [can not add]
nostil_p@yahoo.xom [can not add]
hkremsik@hotmail.xom [Already a Hotmail Contact]