Using a Microsoft Azure VM as a DAG witness server
Exchange Server 2013 enables you to configure your mailbox databases in a database availability group (DAG) for automatic datacenter failover.
This configuration requires three separate physical locations: two datacenters for mailbox servers and a third location to place the witness server for the DAG.
Organizations with only two physical locations now can also take advantage of automatic datacenter failover by using a Microsoft Azure file server virtual machine to act as the DAG’s witness server.
Taken from the site of Thanks technet