It never really existed in the Outlook Address Book, only in Outlook Express
and Outlook IMO.
To automatically add the email address of people
you respond to to your Contacts Folder requires code:

http://www.outlookcode.com/d/code/autoaddrecip.htm
http://www.slipstick.com/contacts/addauto.htm

Hi keithcat,

you could try also our freeware addins for Outlook: Public Contact After
Reply for Outlook:

Public Contact after Reply for Outlook (Public CAR for Outlook) is a
freeware addin for Outlook, which create automatically an Outlook contact
after reply to an email.
http://www.publicshareware.com/outlook-public-contact-after-reply.html

or for existing emails: Public Mail 2 Contact for Outlook (freeware)

Public Mail 2 Contact is a freeware addin for Outlook  It generates Outlook
contacts from existing e-mails and verifies whether the e-mail-address does
already exist as contact in Outlook.
http://www.publicshareware.com/outlook-public-mail-to-contact.html

also available on the microsoft server:
http://office.microsoft.com/en-us/marketplace/EM101932941033.aspx?CategoryID=CE0
11869211033

Maybe it helps.

--
Oliver Vukovics
Share Outlook without Exchange: Public ShareFolder
Freeware addin for Outlook: Public Mail 2 Contact
http://www.publicshareware.com