An entity that is located in the message transfer layer, coresident with a UA and not with an MTA, and responsible for controlling the submission and delivery interactions with an message transfer agent entity.; 一種處于報文傳送層的實體,與用戶代理共存,不與報文傳送代理共存,負責控制與報文傳送代理實體的托 付和交付交互作用。