SMTP Service Extension for Message Size Declaration (RFC1427)
Original Publication Date: 1993-Feb-01
Included in the Prior Art Database: 2019-Feb-10
Internet Society Requests For Comment (RFCs)
J. Klensin: AUTHOR [+3]
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. [STANDARDS-TRACK]
Network Working Group J. Klensin, WG Chair Request for Comments: 1427 United Nations University N. Freed, Editor Innosoft International, Inc. K. Moore University of Tennessee February 1993
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:
Klensin, Freed & Moore [Page 1]
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 :
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 :
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...