The encoder (FormMailEncoder) is a binary program - it does not rely on PHP. We have versions that will run on: Linux, FreeBSD, Windows, Sun Solaris.
In general, we can also provide versions for other server types.
You should be able to use it with your CGI version of FormMail, but that script may need modification in order to know how to access FormMailEncoder.
Very shortly you'll be able to download a free trial version for FormMailEncoder/Decoder.
However, in the interim, I've attached the Help file for FormMailDecoder. You can download and open this on a Windows PC. It contains the full documentation for getting FormMailEncoder/Decoder configured and running.
If you have any questions or would like any further assistance, please let me know.