A name that has been verified is underlined. If Outlook can't find a properly formatted e-mail address to go with a name, it prompts you to find one.
The Address Book has uses beyond helping you find an address. When you type a person's name on the To line of a message, Outlook uses the Address Book to look up and verify that the e-mail address listed for that person is properly formatted before it sends the message. Outlook will show that it has verified the address by underlining the name on the To line. (This verification process is sometimes called "resolving" the name.)
To have Outlook quickly resolve a name, click the Check Names button in the message, or press CTRL+K. After you perform one of these actions, notice that the name is underlined to indicate that it is resolved. If a name can't be mapped to an e-mail address in your Address Book (and if it doesn’t resemble an Internet address), Outlook will prompt you to verify it with the Check Names dialog box. This dialog box will also appear to help ensure that you send e-mail to the correct person — for example, in the case where you have two similarly named people in your Address Book (in this case you might see a red squiggly line under the name on the To line).
Note If you type a complete e-mail address, for example, "firstname.lastname@example.org," Outlook will automatically resolve it. Why? Because Outlook automatically accepts as valid any address that resembles an Internet e-mail address, also called a Simple Mail Transfer Protocol (SMTP) address.