Google does a little of this already although the mechanism is not as direct as your version. E-mail from certain senders gains "importance" based on your inter

Google does a little of this already although the mechanism is not as direct as your version. E-mail from certain senders gains "importance" based on your interactions with that sender, such as if you'd first sent a mail to that address, if you'd ever replied to that address, if you open a certain amount of mail from that address, etc. Mail from senders considered "important" is then more likely to hit your inbox. It seems to work reasonably well, although there are some interesting ways you can game it. One I learnt from the Internet marketing world was some list builders (using legit methods, but perhaps promoting things that often get caught by spam filters) hire people or implement techniques to encourage new list signups to reply to mails sent from the same address as the list by asking them questions, etc.