If you have reattached or moved your outlook file you may find that when you type a name in the To.. email address field the contact email address is not found.
This will be because outlook has its address book mapped to an outlook contact list that no longer exists.
In order to remap the address book to the current one follow the procedure below:
In outlook go to tools/E-mail Accounts
Select View or change existing directories or address books
Remove the Outlook Address Book
Immediately Click Add.. after it has been removed and select Additional Address Books
Select next and finish
Close and reopen outlook and the address book should have been reset to the local contacts.
See this procedure for adding a public folder address book to the addresses searched when sending an email