root# cd /etc/postfix
root# grep cidr main.cf 
                        check_sender_mx_access cidr:/etc/postfix/access/sender_ip_access
                        check_sender_ns_access cidr:/etc/postfix/access/sender_ip_access
root# file /etc/postfix/access/sender_ip_access
/etc/postfix/access/sender_ip_access: ASCII C++ program text
root# ll /etc/postfix/access/sender_ip_access
-rw-r--r--  1 root root 674 Jul 29 13:36 /etc/postfix/access/sender_ip_access
root# postmap cidr:/etc/postfix/access/sender_ip_access
postmap: fatal: dict_mkmap_func: unsupported dictionary type: cidr:  Is the postfix-cidr package installed?
root#