Recently I worked on an Exchange 2007 to Exchange 2013 on premise migration. We had cut over the client access services from Exchange 2007 to 2013. Client Access was published externally via IIS ARR. I used this post by my good friend (and Lync Guru) Yoav Barzilay to configure IIS ARR. Although the post is for Lync, it is a great guide that will also apply to Exchange.