One of the four core components of Microsoft Exchange Server. The Message Transfer Agent (MTA) routes messages between servers, sites, and foreign mail systems. It is implemented as a Microsoft Windows NT service on an Exchange server. The MTA is responsible only for delivering messages from one Exchange server to a different server. It is not responsible for delivering messages between users whose mailboxes are located on the same Exchange server.
The term “message transfer agent” is also used more generally to describe any mail system software that can move messages from one message store location to another. For example, the External program for the legacy Microsoft Mail version 3 could move messages from one Microsoft Mail postoffice to another. MTAs also form a core component of any X.400 messaging system.