saop adds authentication to any smtp server, using a pop3 or imap4 server as the authentication backend.
To do so, it sits in the middle of an SMTP session and intercepts login attempts. It then captures the user and password and uses them to authenticate against a POP3 server, an IMAP4 server or a password list.
It was created for the IIS SMTP service on windows, although the code compiles and works on linux (even better than on windows). In the case of IIS, it allows the SMTP service to authenticate users with the same user/password as the POP3 service without having to define local users.
I've tested it with postfix and dovecot and with IIS' SMTP and POP3 service, but it should run with any server combinations. On windows it installs as a service and runs at startup.
saop doesn't have any dependencies apart from a pthread library. On Linux it should be included as part of glibc, but on windows you'll need to download pthreads-w32 separately in case you want to compile it. It's included on the installer, so if all you want to do is execute saop, then just download the installer and you are good to go.
Documentation is included in the distribution file and an example of a configuration file is included.
saop is licensed under the GPL