The unknown-8bit codec is specified in MIME as something one may use when sending 8-bit text of unknown provenance.
This codec maps the upper 128 code points to the Unicode private use area, U+ED80 to U+EDFF. This section of the private use are was chosen at random (except that we googled to see if anyone else to was using it). 0 is mapped to U+ED00 since some people actually do send null bytes, and at least one MTA accepts then. If any of 1-127 should be unknown rather than ascii, we'll use the remaining EDxy code points for that.
This web page based on source code belonging to The Archiveopteryx Developers. All rights reserved.