PMDF System Manager's Guide

Messages sent by PMDF are received in an encoded format; e.g.,

Date: Sun, 07 Jul 2012 11:59:56 -0700 (PDT) From: "Elvis Presley" <elvis@example.com> To: priscilla@example.edu Subject: test message with 8bit data MIME-Version: 1.0 Content-type: TEXT/PLAIN; CHARSET=ISO-8859-1 Content-transfer-encoding: QUOTED-PRINTABLE 2=00So are the Bo=F6tes Void and the Coal Sack the same?=

Such messages appear unencoded when read with a MIME-aware user agent such as Pine or when decoded with a decoder such as pmdf decode.

The SMTP protocol as set forth by RFC 821 only allows the transmission of ASCII characters. As ASCII is a seven-bit character set, the transmission via SMTP of eight bit characters is illegal. As a practical matter, the transmission of eight bit characters over SMTP is known to cause a variety of problems with some SMTP servers (e.g., cause SMTP servers to go into compute bound loops, cause mail messages to be sent over and over again, crash SMTP servers, wreak havoc with user agents or mailboxes which cannot handle eight bit data, etc.).

Until the advent of RFC 1425 and RFC 1426, an SMTP client had only three alternatives when presented with a message containing eight bit data: return the message to the sender as undeliverable, encode the message, or send it anyhow in direct violation of RFC 821. None of these alternatives were pleasant; prior to version 4.2, PMDF chose the latter of the three owing to the lack of a standardized encoding format. However, with the recent advent of MIME (first specified in RFCs 1521 and 1522, and updated in RFCs 2045--2049) and the SMTP extensions work (RFC 1425 and RFC 1426), there are now standard encodings which can be used to encode eight bit data using the ASCII character set and mechanisms to negotiate, between the SMTP client and server, whether or not eight bit data will be accepted as is by the server without first being encoded.

When recipients receive encoded messages such as those shown above with a MIME content type of TEXT/PLAIN, then invariably the original message contained eight bit characters and the remote SMTP server to which the PMDF SMTP client transferred the message did not support the transfer of eight bit data. PMDF then had to encode the message.