Great news for Exchange Administrators was given by MSExchange Team where now we can use an Azure VM as DAG Witness Server.
I’m happy to announce support for use of an Azure virtual machine as an Exchange 2013 Database Availability Group witness server. Automatic datacenter failover in Exchange 2013 requires three physical sites, but many of our customers with stretched DAGs only have two physical sites deployed today. By enabling the use of Azure as a third physical site, this provides many of our customers with a cost-effective method for improving the overall availability and resiliency of their Exchange deployment.
You can learn more about the deployment and configuration process, as well as learn about our best practices in the TechNet Library article.
Read more at source: http://blogs.technet.com/b/exchange/archive/2015/01/09/using-an-azure-vm-as-a-dag-witness-server.aspx