How can I associate an email sent or received from a contact with a different contact?

Foe Example: I want to send an email to John Smith. The content of the email concerns another contact, Mary Jones, but I do NOT want to send the email to Mary Jones. I do want this email and any replies to be associated with both contacts though so I can keep them straight within the deals, accounts, and contacts modules. I have looked, but can't find an answer anywhere. Thank you in advance for your reply.

Lance