TCP/IP Tutorial and Technical Overview - IBM Redbooks

RFC Description
3030 This introduces two extensions. The first allows SMTP clients to use a BDAT
command as an alternative to the DATA command (see Figure 15-2 on
page 564). This provides the ability to efficiently send large MIME
messages.
Note that the RFC 1652 and MIME approaches are complementary rather than
competing standards. In particular, RFC 1652 is titled “SMTP Service Extension
for 8-bit-MIMEtransport,” because the MIME standard allows messages to be
declared as consisting of 8-bit data rather than 7-bit data. Such messages
cannot be transmitted by SMTP agents that strictly conform to RFC 2821, but
can be transmitted when both the client and the server conform to RFC 1652.
Whenever a client SMTP attempts to send 8-bit data to a server that does not
support this extension, the client SMTP must either encode the message
contents into a 7-bit representation compliant with the MIME standard or return a
permanent error to the user.
Additionally, the service extension defined in RFC 1652 does not permit the
sending of arbitrary binary data, because RFC 2821 defines the maximum length
of a line that an SMTP server is required to accept as 1000 characters. However,
non-text data can easily have sequences of more than 1000 characters without a
sequence.
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The second extension takes advantage of the new BDAT command to
permit sending of MIME messages that employ the binary transfer
encoding.
3207 This introduces the ability for SMTP clients and servers to implement
Transport Layer Security (TLS, see 22.8, “Transport Layer Security (TLS)”
on page 861) in their protocol interactions, the sending of data, or both.
3461 This introduces the ability for SMTP clients to request notifications of
delivery status, allowing clients to know if messages have been successfully
delivered, delayed, and so on.
3885 This extends RFC 3461 to allow clients to track the sending of a message
when needed replies described in 3461 are not received.
Note: The service extension specifically limits the use of non-ASCII
characters (those with values above decimal 127) to message bodies.
They are not permitted in RFC 2822 message headers.