O3 Mail System.

O3

O3 is a fast scalable email system that supports SMTP and POP 4. It is also the reference implementation for POP 4.
As soon as I finish the documentation it will be available for download.

Okay, the documentation isn't finished yet, but you can download the tarball.

There's a README in the tarball and a makefile.

Download the O3 tarball here.

The POP 4 Home page.
Contact information:
I can be contacted a pop4@pop4.org
Please also check sourceforge for project information related to O3 and POP 4.