About automatically checking recipient names before sending a message
Before you send a message, Microsoft Outlook automatically checks the names you type in the To, Cc, and Bcc boxes against the names in the Address Book and against those contact folders that you've specified as Outlook Address Books. If an exact match is found, the name is underlined.
If multiple matches are found and you haven't used the address before, a red, wavy line appears under the name. Right-click the name to see the matches, and then select your choice. If multiple matches are found and you have used the address before, the name you chose previously is displayed in the box with a green, dashed underline to remind you that there are other matches. Right-click the name to see the matches.
You can also manually check names in messages by clicking Check Names 
on the message toolbar.