Intersite Messaging enables messages to be exchanged between computers running Windows Server sites. This service is used for mail-based replication between sites. Active Directory includes support for replication between sites by using SMTP over IP transport. SMTP support is provided by the SMTP service, which is a component of IIS. The set of transports used for communication between sites must be extensible; therefore, each transport is defined in a separate add-in dynamic link library (DLL). These add-in DLLs are loaded into the ISM service, which runs on all domain controllers that are candidates for performing communication between sites. The ISM service directs send requests and receive requests to the appropriate transport add-in DLLs, which then route the messages to the ISM service on the destination computer.
If this service is stopped or disabled, Intersite messaging replication will not work nor will site routing information be calculated for other services.