diff --git a/data/Dockerfiles/postfix/postfix.sh b/data/Dockerfiles/postfix/postfix.sh
index 514fbccb..dffcfd44 100755
--- a/data/Dockerfiles/postfix/postfix.sh
+++ b/data/Dockerfiles/postfix/postfix.sh
@@ -358,7 +358,13 @@ fi
 
 if [[ ! -f /opt/postfix/conf/custom_postscreen_whitelist.cidr ]]; then
   echo "Creating dummy custom_postscreen_whitelist.cidr"
-  echo '# Autogenerated by mailcow' > /opt/postfix/conf/custom_postscreen_whitelist.cidr
+  cat <<EOF > /opt/postfix/conf/custom_postscreen_whitelist.cidr
+# Autogenerated by mailcow
+# Rules are evaluated in the order as specified.
+# Blacklist 192.168.* except 192.168.0.1.
+# 192.168.0.1          permit
+# 192.168.0.0/16       reject
+EOF
 fi
 
 # Fix SMTP last login on slaves