TCP/IP Tutorial and Technical Overview - IBM Redbooks

RFC 2821 dictates that data sent through SMTP is 7-bit ASCII data, with the
high-order bit cleared to zero. This is adequate in most instances for the
transmission of English text messages, but is inadequate for non-English text or
non-textual data. There are two approaches to overcoming these limitations:
► Multipurpose Internet Mail Extensions (MIME), defined in RFCs 2045 to 2049,
which specifies a mechanism for encoding text and binary data as 7-bit ASCII
within the mail envelope defined by RFC 2822. MIME is described in 15.3,
“Multipurpose Internet Mail Extensions (MIME)” on page 571.
► SMTP service extensions, which define a mechanism to extend the
capabilities of SMTP beyond the limitations imposed by RFC 2821. In the
context of overcoming the 7-bit ASCII limitation, the two most important
extensions are defined in RFC 1652 and RFC 1870. However, there are
multiple current RFCs that describe other SMTP service extensions, some of
the more notable of which are detailed in Table 15-1.
Table 15-1 SMTP service extensions
RFC Description
1652 A protocol for 8-bit text transmission that allows an SMTP server to indicate
that it can accept data consisting of 8-bit bytes. A server that reports that this
extension is available to a client must leave the high order bit of bytes
received in an SMTP message unchanged if requested to do so by the
client.
1845 A protocol to allow an interrupted SMTP transaction to be restarted at a later
time. This prevents the need to repeat all of the commands, or retransmit
data that had been successfully received prior to the interruption.
1846 This extension allows an Internet host implementing SMTP to return a 521
message, indicating that it does not accept incoming mail.
1870 A protocol for message size declaration that allows a server to inform a
client of the maximum size message it can accept. Without this extension, a
client can only be informed that a message has exceeded the maximum size
acceptable to the server (either a fixed upper limit or a temporary limit
imposed by a lack of available storage space at the server) after transmitting
the entire message. When this happens, the server discards the failing
message. If both the client and server support the message size declaration
extension, the client can declare an estimated size of the message to be
transferred and the server will return an error if the message is too large.
2034 This extension defines enhanced error codes to allow SMTP the ability to
return more informative explanations of error conditions.
2554 This introduces an option to allow an SMTP client and server to negotiate
an authentication mechanism, and optionally negotiate a security layer to
protect subsequent protocol interactions.
Chapter 15. Mail applications 557