New users cannot open Outlook without changing their mailbox

When new users log on to a PC and open outlook they are always prompted to enter their Microsoft Exchange Server and Mailbox. When they click Check Name they get the error message:

"The name cannot be resolved. The name cannot be matched to a name in the address list".

This is because users log in to their PC using their 6-character Alias; but their mailbox is their

firstname.lastname@school.com

So the Outlook PRF automatically populates the Mailbox with the Alias instead of the mailbox.

How do I change the PRF so it populates the Mailbox with the e-mail address instead of the Alias?

Or do I need to change Autodiscovery to the mail address instead of the Alias?

More information:-

We have an federated Office 365; A.D on-site is synced with Office 365 so their accounts are created on-premise, synced with the cloud and a mailbox created in the Office365 cloud for them

Exchange 2010 on-site; A.D is Server 2008 R2. All desktops are Windows 7.