Contacts sometimes sign up for a new email address. That doesn't mean that emails they sent to me from an old address are no longer from that contact. It just means they don't use that address any more.
I want to keep those old addresses associated with a contact. I just don't want to accidentally send emails to the old address because Gmail autofilled it in.
Is there any way to deprecate an old e-mail address so Gmail stops autofilling it in the To: box, but still knows that emails from that address came from that person?