postfix and CIDR blocklisting
I have discovered the joy of Postfix CIDR block lists. If you run Postfix, and want to block all mail from China and Korea, here's what you need to do (as root):
That's it. If you want to update the list of IP addresses, just repeat steps 1-3. For testing purposes you can insert "warn_if_reject" before "check_client_access". This will allow the mail to be delivered, but write a reject_warning message to /var/log/maillog.
(You should probably run "postconf -m" and verify that "cidr" is in the list of supported table types.)