4.2. The "Q" encoding

The "Q" encoding is similar to the "Quoted-Printable" content- transfer-encoding defined in RFC 1521. It is designed to allow text containing mostly ASCII characters to be decipherable on an ASCII terminal without decoding.

1. Any 8-bit value may be represented by a "=" followed by two hexadecimal digits. For example, if the character set in use were ISO-8859-1, the "=" character would thus be encoded as "=3D", and a SPACE by "=20". (Upper case should be used for hexadecimal digits "A" through "F".)

2. The 8-bit hexadecimal value 20 (e.g., ISO-8859-1 SPACE) may be represented as "_" (underscore, ASCII 95.). (This character may not pass through some internetwork mail gateways, but its use will greatly enhance readability of "Q" encoded data with mail readers that do not support this encoding.) Note that the "_" always represents hexadecimal 20, even if the SPACE character occupies a different code position in the character set in use.

3. 8-bit values which correspond to printable ASCII characters other than "=", "?", "_" (underscore), and SPACE may be represented as those characters. (But see section 5 for restrictions.)