As you know, the Microsoft Exchange Autodiscover service provides an easier option to configure Outlook 2007 or Outlook 2010 and some mobile phones. The Autodiscover process looks for the primary SMTP address (mail) of the user object to obtain the Exchange server information. If you are using an exchange Resource forest, you need to make sure the primary SMTP address is populated in your account forest also. I have seen many reference about FQND of the computer, DNS suffix etc. However, based on my testing and experience, Autodisover will only work, if you have primary SMTP address populated in the account forest. You don’t need to extend the schema, you can just add the mail address in the mail attribute.
I have created a script which can be used to copy email address from a Resource to Account forest - http://portal.sivarajan.com/2013/08/copy-smtp-address-from-exchange.html if you don’t have any other solutions in-place.
If you are performing a migration, you may have different options to populate this value. On my current migration project, I am using Quest Migration Manager (QMM). In QMM, you can use a custom add-ins to update the email address in the account forest.
Please keep in mind that if you are using Quest Client Profile Update Utility / EMPWProf, you may run into a conflict with Exchange Autodiscover service. You can read more details in this Quest solution article (Knowledge Article 51188) - https://support.quest.com/SolutionDetail.aspx?id=SOL51188&pr=Migration Manager for Exchange
Migrating from Windows Server 2008 or Windows Server 2008 R2 to Windows Sever 2012?