A protocol entity forming part of the application layer. It provides a generalized facility for exchanging electronic message between systems. It is also known as X.400.; 一種構(gòu)成應(yīng)用層中一部分的協(xié)議實體。 該協(xié)議實體在系統(tǒng)之間提供交換電子報文的通用化設(shè)施。報文處理服務(wù)層也稱為X.400。