When I check my email, I scan the list of new messages for names of people I know.
This is something that machines can do much faster and better. With the volume of spam and bulk mail these days, some of the vast effort spent on reducing spam should be spent on showing users the good stuff.
You can do this the hard way today with many mail services by creating filters for each person you want to highlight (or move to a different folder). This is usually a multi-step process; few people will create and update these filters.
There’s a simple, high value way to accomplish this: Check the messages against the email addresses the user has sent email to and the user’s address book, then highlight the ones that match.
If the user has categorized the address book, you could also color code the highlighting to indicate whether the message is from friends, family, co-workers, etc.
Rakesh Agrawal's blog 
Another idea only these lines: automatically prioritize threads that I’ve responded to. This could be useful for mailing lists, which I normally filter away.
Actually some email programs do this. uReach.com highlights emails that are from users in the address book and provides icons to represent emails that are from (un)known users, they even have a nice filter to only show emails based on this characteristic.
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