Qsmtp is a drop in replacement for the qmail SMTP programs qmail-smtpd and qmail-remote. That means you just change your startup scripts to use Qsmtpd instead of qmail-smtpd and everything will work as before (but you will get the new features). Be careful: Qsmtpd does only look at the standard tcpserver environment variables, setting RELAYCLIENT will not work. You can use /var/qmail/control/relayclients and /var/qmail/control/relayclients6 to get the same behaviour (this is not documented yet, take a look at targets/addipbl.c to see how it works). If you want to use Qremote instead of qmail-remote just move your qmail-remote binary out of the way and create a symlink from Qremote to qmail-remote.
But be careful: this software is still under development and may crash, kill your mails or eat up all your memory if something goes wrong. But things aren't that bad usually: I have 2 hosts running Qsmtp (both Qsmtpd and Qremote) for more than two years and it looks fine. This are "real" hosts, receiving at least 300 mails per day each from the internet. The total amount of mail connections is about 100,000 per month where the mayority is blocked for one or the other reason.
Qsmtp is released under GNU GPL version 2.
You may ask: why should I use it? The benefits are:
These are the requirements to your system and your installed software to run Qsmtpd:
The latest version is Qsmtp 0.12. You can always get the latest snapshot of the development from the subversion repository at http://opensource.sf-tec.de/repos/Qsmtp/. New releases will be announced on the qsmtp-announce mailing list. You can subscribe by sending an empty mail to qsmtp-announce-subscribe@opensource.sf-tec.de. The installation instructions will help you to get it running.
These are the things I would like to see in Qsmtp sometime. They are in no particular order. If you would like to see them quickly feel free to send money or patches.