Exchange is designed to meet the ACID (atomicity, consistency, isolation, and durability) test for database transaction integrity. Transactions are only applied if they are compete and intact. Great efforts are made to ensure consistency throughout the Store; transactions are isolated from each other; and there are many features implemented in the Store to make it a reliable and robust repository.

A transaction is a series of modifications to a database that leaves the database in a consistent state. These modifications are called operations. Sending a new message requires several operations that form the single logical transaction that we think of when we think of a message being sent and delivered to recipient mailboxes.

An operation is the smallest change that can be made to an ESE database. Operations include insert, delete, replace, and commit. A database is left in an inconsistent state if not all of the operations that make up a transaction are committed. For example, if a server experiences a sudden failure during normal operation, it is quite likely that a number of transactions are outstanding and have not yet been completely committed to the database. Exchange can return its databases to a consistent state by processing the contents of the transaction log files. Making the database consistent can result in some transactions being ignored because not all operations were completed when the system crash occurred.

ACID transactions mean that ESE must fulfill the following requirements, as described in Transaction Processing: Concepts and Techniques