Exchange Server 2010 uses database availability groups (DAGs) to provide automatic database level recovery from failures of mailbox servers or individual mailbox databases. When one database in a DAG fails, Exchange makes active another passive copy of the database on a different mailbox server.

To ensure that the mailboxes you enable for archiving are always available to Enterprise Vault, you must set up archiving for all the DAG member servers. You must also target all the DAG member servers within one Enterprise Vault site.

It is possible to add an Exchange mailbox archiving task for a server, only when it hosts an active Exchange database. Your environment might contain Exchange 2010 servers that act only as disaster recovery (DR) servers, and do not normally host active DAG member databases. These servers must be set up for Exchange server archiving because active DAG member databases can fail over to them. However, while these servers are not hosting active databases, you cannot set them up for Exchange mailbox archiving.

When all DAG member servers are set up for archiving, database and server failovers do not interrupt mailbox archiving.

See Overview of setting up Enterprise Vault Exchange Server mailbox archiving

See Exchange mailbox archiving and database failovers with Exchange Server mailbox archiving