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.