I tried this already and it's not the case with me
When I click To:, Cc:, or Bcc: I can't see my contacts.
On the Tools menu, click E-mail Accounts.
Select View or change existing directories or address books, then click Next.
This will display if the Microsoft Outlook Address Book (Address Book: The
collection of address books that you can use to store names, e-mail
addresses, fax numbers, and distribution lists. The Address Book may contain
a Global Address List, an Outlook Address Book, and a Personal Address Book.)
is added. Do one of the following:
The Outlook Address Book is listed.
On the Go menu, click Folder List.
Right-click your contact folders, click Properties, and then click the
Outlook Address Book tab.
Make sure Show this folder as an e-mail Address Book is selected.
The Outlook Address Book is not listed.
Click Add, select Additional Address Books, and then click Next.
From the list, select Outlook Address Book, then click Next.
You must restart Outlook to complete this change.