This occurs when you move from a Hosted Exchange 2003 environment to 2013.
To regain the auto populate ability within Hosted Exchange 2013 you will need to follow these few steps.
Please ensure that your outlook is WORKING OFFLINE for this procedure, open the "Send / Receive" Tab and click on the button to "Work Offline":
Confirmation of Working Offline:
1) Open up you contacts within outlook:
2) Choose your suggested contacts:
3) Select all of your contacts (Ctrl and A)
4) On the top menu bar choose the option email:
5) A new email will open up with the "To" field showing all your suggested contacts:
6) Then send the email.
7) Locate the email in your "Outbox" and delete it:
Place your Outlook back Online.
When you now open a new email, on the commencement of typing of one of your suggested contacts you will have the option to choose via auto populate.
Please note: If this does not work it means you have exceeded the number of contacts allowed in the "To" field of an email. Rather than choosing all, you will have to select your suggested contacts in small batches.