Outlookanywhere ExternalHostName

Hello,

I am curenlty preparing the migration from Exchange 2010 to Exchange 2013 and have to set OutlookAnywhere in the proper way. Curently and in the closely feature all clients will use OutlookAnywhere only from the internal network (directly or over vpn), it is a CIT decision. How to configure ExternalHOstName of OutlookAnywhere, can it be blank?

Thank you.