In addition to filtering out mail you don’t want, Mail Rules can highlight mail you consider important. For example, you probably add people to your Address Book because you correspond with them on a regular basis. Here’s how to give messages from these folk top priority in your mailbox.
Choose Rules from the Tools menu and click on New.
In the Define Mail Rule dialog box, give the Rule a name and make sure Enabled is selected .
To apply the rule to mail from anyone in your address book, click on the pop-up menu in the If section and choose From . A new pop-up menu appears to the right; choose Is in address book. You can add additional requirements to your Rule by clicking on Add Criterion . A new row of pop-up menus will appear.
In the Then section, choose Change priority from the pop-up menu . In the new pop-up menu that appears to the right, choose the priority setting you want to assign to these messages. In this case I chose Highest.
If you want to make the messages even easier to spot, give them their own color. Click on the Add Action button  and choose Change Color from the pop-up menu . Pick a color that will grab your attention.
To top off your efforts, open the in-box and click on the Priority button  to sort the in-box by priority. This will list all of your high-priority e-mail messages together. Click on the Priority button again to toggle between sorting in ascending or descending order. To make the Highest priority messages appear at the top of your in-box, sort in descending order.
Now you’ll always find your most important e-mail messages waiting at the top of your in-box.