The directory service maintains all available information about the entire Exchange Organization including. Every server in a site contains a copy of the directory for that site.
Information is replicated between servers.
Within a site the MTA routes messages directly to the MTA on other exchange servers. When delivering mail to foreign systems MTA uses Connectors. MTA is also responsible for converting message formats say from Exchange to X.400 and back again. It is also responsible for transferring messages between users on the same exchange server. The Information Store is both a database and service.