SMTP Service Extension for Message Size Declaration (RFC1427)
Original Publication Date: 1993-Feb-01
Included in the Prior Art Database: 2000-Sep-12
Internet Society Requests For Comment (RFCs)
J. Klensin: AUTHOR [+5]
This memo defines an extension to the SMTP service whereby an SMTP client and server may interact to give the server an opportunity to decline to accept a message (perhaps temporarily) based on the client's estimate of the message size.
Network Working Group J. Klensin, WG Chair
Request for Comments: 1427 United Nations University
N. Freed, Editor
Innosoft International, Inc.
University of Tennessee
SMTP Service Extension
for Message Size Declaration
Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
This memo defines an extension to the SMTP service whereby an SMTP
client and server may interact to give the server an opportunity to
decline to accept a message (perhaps temporarily) based on the
client's estimate of the message size.
The MIME extensions to the Internet message protocol provide for the
transmission of many kinds of data which were previously unsupported
in Internet mail. One expected result of the use of MIME is that
SMTP will be expected to carry a much wider range of message sizes
than was previously the case. This has an impact on the amount of
resources (e.g., disk space) required by a system acting as a server.
This memo uses the mechanism defined in  to define extensions to
the SMTP service whereby a client ("sender-SMTP") may declare the
size of a particular message to a server ("receiver-SMTP"), after
which the server may indicate to the client that it is or is not
willing to accept the message based on the declared message size and
whereby a server ("receiver-SMTP") may declare the maximum message
size it is willing to accept to a client ("sender-SMTP").
3. Framework for the Size Declaration Extension
The following service extension is therefore defined:
(1) the name of the SMTP service extension is "Message Size
(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 :
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
(4) one optional parameter using the keyword "SIZE" is added to the MAIL
FROM command. The value associated with this parameter is a decimal