Dears is we can use MRS between two servers remotly and when main server down is we can apply users to open his emails from remote server lets say by using web base at least??

What MRS Service and version of Exch are you using? Depending on your deployment the answer varies.Sukh

Hi, How many Exchange Servers in the network? Which roles have been installed? Have you deployed DAG for Mailbox role,Cas Array for CAS role? If we have DAG implemented, then when site down, it will failover to the other site. We need to do some DNS settings to point the A record to the CAS in the other site so that Owa user can still access to their mailbox. Also we need to change rpcclientaccess settings on the Mailboxdatabase. Xiu Zhang TechNet Community Support