The InterOrg Synchronization Tool lets multiple Exchange Server organizations synchronize the contents of their directories. The connector works hierarchically. A master service runs in the master organization, and multiple requestor services run in the branch organizations. The Exchange server of the master organization collects and distributes all the directory changes from the branch organizations. The requestor service in each branch organization collects any changes that have occurred in the directory of that organization and exports these changes to the master service. At the same time, the requestor receives the updated directory information from the master service. Upon receiving updates from a requestor, the master agent creates a new entry and assigns it an update sequence number (USN) that is greater than the USN for the last entry processed. Each requestor stores the largest master USN that it has received, so it requests only new entries to be posted to the global directory. The master and requestor services communicate using the Exchange Internet Mail Service (IMS). Therefore, you must have an SMTP infrastructure in place before you configure this service.
On a scheduled basis, the requestor service generates a mail message for the master. This message includes any changes that have been made to the requestor organization's directory, and the requestor's most recent (i.e., highest) master USN. The requestor exports only the directory information that is maintained on the General property page of an address book entry. That is, the requestor service doesn't export the information available in any of the custom attributes.
When the master receives a mail message from a requestor, it updates the master database with the appropriate changes, then sends a reply message to the requestor. The message to the requestor contains all updates to the master database that have a higher USN than the one that the requestor sent. Although the master agent processes all requestor messages immediately, changes might not replicate throughout the entire system for at least 24 hours.