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   TCP Transport service

      The Transmission Control Protocol [3] is used in the ARPA
      Internet, and in any network following the US DoD standards for
      internetwork protocols.

      Connection Establishment

         The SMTP transmission channel is a TCP connection established
         between the sender process port U and the receiver process port
         L.  This single full duplex connection is used as the
         transmission channel.  This protocol is assigned the service
         port 25 (31 octal), that is L=25.

      Data Transfer

         The TCP connection supports the transmission of 8-bit bytes.
         The SMTP data is 7-bit ASCII characters.  Each character is
         transmitted as an 8-bit byte with the high-order bit cleared to

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