XCON: A Description of MIME Content Transfer Encoding Binary

A message that is sent from an Exchange 2000 computer may use a binary message encoding.

Exchange 2000 uses binary transfer encoding only to encode e-mail messages that are sent in a routing group. Binary transfer encoding has no line-length limitations. This type of encoding is used for the following reasons:

• To avoid the overhead of encoding in base64 and Quoted printable when large multiple-media data files are sent.
• To take advantage of the lack of strict enforcement of Carriage Return LineFeed (CRLF) semantics for binary messages.

When Exchange 2000 encodes a message as "binary," it typically indicates that the message was sent from one server in a routing group to another server in the same routing group in what is also known as Summary Transport Neutral Encapsulation Format (TNEF).

Binary messages have the following characteristics:

• They may include non-ASCII characters outside the 0-127 US-ASCII range.
• The lines in these messages do not have to be short enough for SMTP transport.
• They do not follow the CRLF.CRLF line termination semantics as specified in Request for Comments (RFC) 821.

SMTP servers that decide to send Binary data to servers that advertise the ESMTP verb "BINARYMIME" have to do this by using the MAIL FROM:<user@domain.com>"BODY=BINARYMIME " ESMTP extension.

Binary encoded messages are not valid Internet messages. Exchange 2000 does not send MIME-formatted messages with binary transfer encoding to an Internet host because RFC 2821 restricts mail messages to 7-bit US-ASCII data with lines that are no longer than 998 characters. Additionally, messages that are encoded with the "BINARY" type are sent as a TNEF binary large object in a Winmail.dat attachment. The rich text properties of the message are extracted from this TNEF binary large object to decode the binary message.

The following data is an example of a binary encoded message: