flush rules on firewall teardown

postfix.nix

@@ -9,6 +9,14 @@
     iptables -I OUTPUT -m tcp -p tcp --dport 25 -d 127.0.0.1 -j ACCEPT
     ip6tables -I OUTPUT -m tcp -p tcp --dport 25 -d ::1 -j ACCEPT
   '';
+  # The following is necessary to prevent the above rules from being added at every nixos-rebuild switch.
+  # See link for more info
+  # https://github.com/NixOS/nixpkgs/issues/201614
+  # Flush the firewall rules
+  networking.firewall.extraStopCommands = ''
+    iptables -F
+    ip6tables -F
+  '';
 
   services.postfix = {
     enable = true;