Although the Microsoft Exchange Server MTA guarantees a high degree of reliability (particularly in a homogenous environment), it cannot guarantee that replication messages or confirmation notifications will not be lost due to hardware failure or other problems. This is particularly true where public folder replicas are configured to reside in different sites and a foreign system is used for MTA connectivity between the sites. The process by which the information store compensates for lost messages is called backfilling.
Backfilling is the process of acquiring information that was lost from an information store as a result of one of the following circumstances:
A server needs to be backfilled when it has not successfully received all the replica messages from other information stores.
When an information store fails to receive complete replica information, it generates a backfill entry, which serves as a placeholder to indicate that information is missing. After waiting for a specified amount of time to receive the missing folder content packet, the information store issues a backfill request to two other servers chosen by a predetermined algorithm. The backfill request indicates what folder content packet is missing and requests that another information store send the information. Any information store can respond to a backfill request, regardless of whether it was the source of the original folder content packet.