Messages sent by the MTA are received in an encoded format. For example:
Date: Wed, 04 Jul 2001 11:59:56 -0700 (PDT) From: "Desdemona Vilalobos" <Desdemona@sesta.com> To: email@example.com 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?=
These messages appear unencoded when read with the MTA decoder command imsimta decode. Refer to the Sun Java System Messaging Server Administration Reference for more information.
The SMTP protocol only allows the transmission of ASCII characters (a seven-bit character set) as set forth by RFC 821. In fact, the unnegotiated transmission of eight-bit characters is illegal via SMTP, and it is known to cause a variety of problems with some SMTP servers. For example, SMTP servers can go into compute bound loops. Messages are sent over and over again. Eight-bit characters can crash SMTP servers. Finally, eight-bit character sets can wreak havoc with browsers and mailboxes that cannot handle eight-bit data.
An SMTP client used to only have three options when handling a message containing eight-bit data: return the message to the sender as undeliverable, encode the message, or send it in direct violation of RFC 821. But with the advent of MIME and the SMTP extensions, there are now standard encodings which may be used to encode eight-bit data by using the ASCII character set.
In the previous example, the recipient received an encoded message with a MIME content type of TEXT/PLAIN. The remote SMTP server (to which the MTA SMTP client transferred the message) did not support the transfer of eight-bit data. Since the original message contained eight-bit characters, the MTA had to encode the message.