The term User Agent (UA) is taken from CCITT X.400 Message Handling Systems (MHS) Recommendations, which define it as follows: "In the context of message handling, the functional object, a component of MHS, by means of which a single direct user engages in message handling." In the Internet environment, programs such as rand mh and Gnu emacs rmail are UAs. UAs exchange messages by calling on a supporting Message Transfer Service (MTS), e.g., the SMTP mail relays used in the Internet.