python-libmilter started as a project to maximize the portability of libmilter. I know about, and have used, the excellent CPython implementation by Stuart D. Gathman py-milter. Though I liked this library for it's thin wrapper around the C libmilter library (and thereby speed), I had some issues with getting it to install, having to find and download the appropriate source code for libmilter version installed, etc. This is why I went with a ground up implementation of libmilter in pure Python. I want something I could just “drop in” to a project and have a milter up and running in almost no time, no matter what OS environment I was in.

I also wanted some more choices for getting things done than the default thread approach. This is why I modularized the milter itself to working as a traditional threading milter wherein each client connection gets its own thread, but I also added factories for single thread asynchronous operation and forking. Do note that the threaded version of python-libmilter is constrained by the GIL, whereas py-milter is not since the threading is implemented in C. If you really want to maximize speed, py-milter might be for you, but be aware that I've been able to get excellent performance from this library in production environments with very heavy processing load (evaluating many regular expressions) using the forking factory.