The Message Transfer Layer (MTL) is a sublayer in the application layer of an MHS system that is specified according to X.400
and represents one of two sublayers in the application layer. The MTL layer creates the service elements for the Message Transfer Service (MTS). These service elements are provided using the services of the underlying layers and the functionality of the instances in the layer. MTL provides access to the transfer services over the network. The second sublayer, which resides above MTL, is the User Agent Layer (UAL).