From KB 2682333:

Outlook also adds a folder to Contacts labeled Suggested Contacts. Addresses get added to this folder as you send or reply to messages addressed to people who don’t exist in your Contacts folder. Outlook doesn't look in Suggested Contacts for names during the AutoComplete process. It only stores addresses that you might want to add to your Contacts. You can double-click an entry in Suggested Contacts and a Contact Form opens that allows you to save it to your Contacts Folder.