RFC 1870 (rfc1870) - Page 3 of 9


SMTP Service Extension for Message Size Declaration



Alternative Format: Original Text Document



RFC 1870                 SMTP Size Declaration             November 1995


   A client using the unextended SMTP protocol defined in [1], can only
   be informed of such failures after transmitting the entire message to
   the server (which discards the transferred message).  If, however,
   both client and server support the Message Size Declaration service
   extension, such conditions may be detected before any transfer is
   attempted.

   An SMTP client wishing to relay a large content may issue the EHLO
   command to start an SMTP session, to determine if the server supports
   any of several service extensions.  If the server responds with code
   250 to the EHLO command, and the response includes the EHLO keyword
   value SIZE, then the Message Size Declaration extension is supported.

   If a numeric parameter follows the SIZE keyword value of the EHLO
   response, it indicates the size of the largest message that the
   server is willing to accept.  Any attempt by a client to transfer a
   message which is larger than this limit will be rejected with a
   permanent failure (552) reply code.

   A server that supports the Message Size Declaration extension will
   accept the extended version of the MAIL command described below.
   When supported by the server, a client may use the extended MAIL
   command (instead of the MAIL command as defined in [1]) to declare an
   estimate of the size of a message it wishes to transfer.  The server
   may then return an appropriate error code if it determines that an
   attempt to transfer a message of that size would fail.

5.  Definitions

   The message size is defined as the number of octets, including CR-LF
   pairs, but not the SMTP DATA command's terminating dot or doubled
   quoting dots, to be transmitted by the SMTP client after receiving
   reply code 354 to the DATA command.

   The fixed maximum message size is defined as the message size of the
   largest message that a server is ever willing to accept.  An attempt
   to transfer any message larger than the fixed maximum message size
   will always fail.  The fixed maximum message size may be an
   implementation artifact of the SMTP server, or it may be chosen by
   the administrator of the server.

   The declared message size is defined as a client's estimate of the
   message size for a particular message.








Klensin, et al              Standards Track