Exchange server 2010 DAG failover

Hi team ,

We have configured exchange server 2010 in DAG environment .

We have added 3 mailbox server in DAG . but my active mailbox copy failed the database are failover to another passive copy server & status is mounted . after failover emails service are not working.

Note : We have also configured NLB on CAS server .All exchange servers placed  in single AD site

Please suggest