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.

At some point, an local SMTP server (QMail) responded really slow on connection attempts. There was no traffic between the client and server so it was a bit unclear why it was locked for about 30 seconds until the so-called '220' response.

Google suggested me (indirectly) to disable reverse DNS lookups on the tcpserver (with the -R, -P, -l 'hostname') flags. But that didn't help.