RFC 1427 (rfc1427) - Page 2 of 8

SMTP Service Extension for Message Size Declaration

RFC 1427                 SMTP Size Declaration             February 1993


(1) the name of the SMTP service extension is "Message Size
    Declaration";

(2) the EHLO keyword value associated with this extension is "SIZE";

(3) one optional parameter is allowed with this EHLO keyword value, a
    decimal number indicating the fixed maximum message size in bytes
    that the server will accept.  The syntax of the parameter is as
    follows, using the augmented BNF notation of [2]:

        size-param ::= [1*DIGIT]

    A parameter value of 0 (zero) indicates that no fixed maximum
    message size is in force.  If the parameter is omitted no
    information is conveyed about the server's fixed maximum message
    size;

(4) one optional parameter using the keyword "SIZE" is added to the MAIL
    FROM command.  The value associated with this parameter is a decimal
    number indicating the size of the message that is to be transmitted.
    The syntax of the value is as follows, using the augmented BNF
    notation of [2]:

        size-value ::= 1*DIGIT

(5) no additional SMTP verbs are defined by this extension.

   The remainder of this memo specifies how support for the extension
   affects the behavior of an SMTP client and server.

4.  The Message Size Declaration service extension

   An SMTP server may have a fixed upper limit on message size.  Any
   attempt by a client to transfer a message which is larger than this
   fixed upper limit will fail.  In addition, a server normally has
   limited space with which to store incoming messages.  Transfer of a
   message may therefore also fail due to a lack of storage space, but
   might succeed at a later time.

   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



Klensin, Freed & Moore