RFC 1870 Internet Standard Transport

SMTP Service Extension for Message Size Declaration

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]

Status
Internet Standard. The top of the standards track. An old document can hold this label while the document that replaced it holds a lower one, which is the single most misleading thing about RFC status.
Published
November 1995
Authors
J. Klensin, N. Freed, K. Moore
Read it
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Replaces RFC 1427: SMTP Service Extension for Message Size Declaration, RFC 1653: SMTP Service Extension for Message Size Declaration.

What it is

The SIZE extension, which lets a server advertise its maximum message size and lets a client declare the size up front.

What it solves

Without it a client transmits an entire message before discovering it is too large, wasting the transfer on both sides.

What it means to operate

Worth honouring on both ends. A sender that ignores the advertised SIZE burns bandwidth and connection slots on messages that will be refused at the end of DATA, and on a busy queue that is throughput you do not get back.

Bounce classifier turns an SMTP response into the action it calls for. Open bounce classifier

Normative requirements

Every sentence in this RFC carrying an RFC 2119 keyword, with the section it came from.

6 The extended MAIL command

  • MUST NOTNOTE: Servers MUST NOT use the SIZE parameter to determine end of content in the DATA command.

Every current email RFC

Written and maintained by Rastu Singh, an infrastructure engineer working on high-volume email in Tallinn.