diff --git a/nixbld-etc-nixos/configuration.nix b/nixbld-etc-nixos/configuration.nix
index 3927092..352f9bd 100644
--- a/nixbld-etc-nixos/configuration.nix
+++ b/nixbld-etc-nixos/configuration.nix
@@ -1261,6 +1261,23 @@ in
     enablePop3Ssl = true;
     certificateScheme = "acme-nginx";
   } // (import /etc/nixos/secret/email_settings.nix);
+  services.postfix = {
+    mapFiles."sender_transport" = builtins.toFile "sender_transport" ''
+        @m-labs-intl.com intltunnel:
+    '';
+    config = {
+      sender_dependent_default_transport_maps = "hash:/var/lib/postfix/conf/sender_transport";
+    };
+    masterConfig."intltunnel" = {
+      type = "unix";
+      command = "smtp";
+      args = [
+        "-o" "inet_interfaces=10.47.3.1"
+        "-o" "smtp_helo_name=mail.m-labs-intl.com"
+        "-o" "inet_protocols=ipv4"
+       ];
+    };
+  };
   services.roundcube = {
     enable = true;
     hostName = "mail.m-labs.hk";