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11. Message Format

All messages transmitted during a POP3 session are assumed to conform to the standard for the format of Internet text messages [RFC822].

It is important to note that the byte count for a message on the server host may differ from the octet count assigned to that message due to local file system conventions regarding line terminators. Usually, during the AUTHORIZATION state of the POP3 session, the POP3 server can calculate the size of each message in octets when it opens the maildrop. For example, if the POP3 server host internally represents end-of-line as a single character, then the POP3 server simply counts each occurrence of this character in a message as two octets. Note that lines in the maildrop which start with the termination octet need not (and must not) be counted twice, since the POP3 server does not byte-stuff when it sends the message to the client.

Key Points:

  • Message Format: All messages conform to RFC 822 (Internet Message Format)
  • Byte Counting: Server must convert local line endings to network format (CRLF) for counting
  • Line Terminator: Network format uses CRLF (2 octets)
  • Byte-Stuffing: Not counted in message size calculation

Related Standards:

  • RFC 822: Standard for ARPA Internet Text Messages
  • RFC 5322: Internet Message Format (updates RFC 822)
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