X.400 The ITU-T standard Message Handling Service Protocol (MHS or MHSP) for e-mail and docu- ment exchange, X.400 is a complex gateway protocol that permits disparate e-mail systems to interoperate at a minimal level, over either packet networks or asynchronous dial-up circuits. Created in 1984 and updated in 1988 and again in 1992, X.400 functions at Layer 7, the Application Layer, of the OSI 74570c24.qxd 9/11/07 12:37 PM Page 549 |