In Outlook 2010 "Suggested Contacts" feature automatically keeps track of everyone you send a message to. Unlike the Auto-Complete List that appears when you begin typing a name or e-mail address in the To, Cc, or Bcc boxes of a message, you can search your suggested contacts, and there's no maximum number of suggested contacts (the Auto-Complete List has a maximum of 1,000 entries).
Here's how to find a lost contact: "In the Navigation Pane, click Contacts. Then click Suggested Contacts folder. In search box near the top enter @searchingdomain, if you know your contact's email address @domain. Once you found, you can drag your contact business card from Suggested Contacts to your Outlook Contacts folder for future reference.