What the NATATA Anti-Spam Encoder Does
One can display any alphanumeric character in the ISO-8859-1 character set on a browser on an HTML page, whether your keyboard supports it or not. (This is not the same as the Windows entry of non-keyboard characters in Windows documents via Alt + numeric keypad keys.)
Ordinarily, this character set is used to display characters with tildes, umlauts, UK pound keys, and other things you won't find on a standard keyboard. However, one can use this to display ordinary alphanumeric (AZ, 09, punctuation, and so on) characters. You simply look up the codes in the table to be found under Character Entity References in the HTML Reference Guide (see below) or listed on this W3C reference page.
The NATATA utility looks specifically for mailto:address links and transforms them into "character entity reference" links readable by browser but not by simple screenscraping, such as the ones you see directly below and in the screen shots.
Sample Character Entity References
a a b b c c 0 0 1 1 2 2