XCON: A Description of 7-Bit MIME Content Transfer Encoding in Exchange Server (323486)

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When Exchange 2000 sends messages in 7-bit encoding, there has been little or no encoding of the message. The characters in the message are all in the US-ASCII range.

• 7-Bit data
  7-bit data refers to data that is represented by relatively short lines with 998 octets or less between Carriage Return Line Feed (CRLF) line separation sequences. Octets with decimal values greater than 127 are not permitted, and neither are "NULL" octets (octets with a decimal equivalent of 0).
• CRLF
  The Carriage Return (CR) (decimal value 13) and Line Feed (LF) (decimal value 10) octets occur only as part of CRLF line separation sequences.
• UTF-7
  UTF-7 encodes Unicode characters as US-ASCII octets, together with shift sequences to encode characters outside that range. For this purpose, one of the characters in the US-ASCII character set is reserved for use as a "shift" character.

When Exchange Server sends messages to SMTP gateways and computers that cannot handle the whole US-ASCII character set (for example, Extended Binary Coded Decimal Interchange Code [EBCDIC] gateways), UTF-7 is used to encode characters in the US-ASCII character set in a format that all mail systems can accept. An example of this process occurs when reports are sent, such as non-delivery reports (NDRs).

Exchange Server 5.5 sends a message using 7-bit encoding if all the characters in the message are in the US-ASCII range. In Exchange 2000, the server encodes a message as 7-bit if character sets from the following code pages or character sets are used to compose the e-mail message: