Not able to add Mailbox server into Database Availability Group in Exchange 2013.
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In Exchange 2013 Microsoft introduced CNO, If your DAG members are running Windows Server 2012, you must pre-stage the CNO prior to adding the first server to the DAG.
The CNO is a computer account created in Active Directory and associated with the cluster's Name resource. The cluster's Name resource is tied to the CNO, which is a Kerberos-enabled object that acts as the cluster's identity and provides the cluster's security context. The formation of the DAG's underlying cluster and the CNO for that cluster is performed when the first member is added to the DAG. When the first server is added to the DAG, remote PowerShell contacts the Microsoft Exchange Replication service on the Mailbox server being added. The Microsoft Exchange Replication service installs the failover clustering feature (if it isn't already installed) and begins the cluster creation process. The Microsoft Exchange Replication service runs under the LOCAL SYSTEM security context, and it's under this context in which cluster creation is performed.
1. Create Computer account (CNO) and disable it.
2. Assign full control of the computer account to the computer account of the first Mailbox server you're adding to the DAG.
3. Assign full control of the computer account to the Exchange Trusted Subsystem USG.
4. Now try to add first exchange mailbox server and followed by second server.