In order to limit the connections made from a certain IP address, one needs to patch tcpserver, the process which receives all incoming connections and passes them through to QMail. These steps describe how to patch tcpserver on a FreeBSD system, where the QMail SMTP and POP servers are executed with the supervise (svc) daemon.

Setting up a POP server with the QMail/VPopmail combination wasn't too much hassle until the point I tried to authenticate.

Telnetting into the POP server and giving authenticating myself resulted in the following error:

When trying to start the QMail POP server with the svc command (daemontools), the following error appeared.

The problem got solved by restarting the daemon: