For Exchange databases, you can perform full backups and incremental backups.
What Is Backed Up
When you back up an Exchange database, the following data is backed up:
Exchange databases and transaction logs
Public Folder databases
What Is Not Backed Up
When you back up an Exchange database, the following data is not backed up:
System state data
Active Directory data
Personal Information Stores (PST files)
Personal Address Books
To back up these items, use the Windows File System Agent.
Database Failures, an Unavailable Database, or Database Failover
In a DAG environment, a database failure can occur during a backup for several reasons (such as the database copy becomes unavailable, or the connection to the network is lost).
When a database failure occurs during a backup job, the backup of the database fails, but the overall backup job continues. During the next incremental backup job, the backup of the failed database is a full backup.
However, when a database failure occurs in the middle of a cycle, the next backup job is an incremental backup.