Qmail - TCPServer
If you're running Qmail using tcpserver to spawn the qmail-smtp process, you can add a rule to your /etc/tcp.smtp file telling it to accept connections based on the reverse DNS of the connecting host:
Qmail - Inetd
If you're running Qmail as an inetd service by using tcp-env then all SMTP access configuration is done from the following files: /etc/hosts.allow and /etc/hosts.deny. Don't forget to add your own SMTP clients to /etc/hosts.allow.
- Add the following line to /etc/hosts.allow
tcp-env: .antispamcloud.com: setenv = RELAYCLIENT
- Add the following line to /etc/hosts.deny . This will deny access from all hosts except those in /etc/hosts.allow for the inetd service qmail
Replace .antispamcloud.com with your servers root hostname.