About truncating Exchange transaction logs with Instant Recovery backups
By default, Exchange transaction logs are not truncated for a full Instant Recovery backup that does not back up to a storage unit. To truncate logs enable in the Exchange properties for the client. Consider carefully before you select this option. Ensure that you have an independent method to retain your snapshots for disaster recovery. Alternatively, you can perform a full Instant Recovery backup to a storage unit.
See Truncating Exchange transaction logs by performing a backup to a storage unit.
For more information on how to configure the client host properties, see the following topic: