fix(loop): reject messages with too many Received headers

config/default.js

@@ -197,6 +197,14 @@ module.exports = {
             addSignatureTimestamp: false,
             // Time validity of the signature given in seconds, for default value see below
             signatureExpireIn: 0
+        },
+
+        // Make sure messages have all required headers like Date or Message-ID
+        'core/delivery-loop': {
+            enabled: ['receiver', 'main'],
+
+            // Reject messages with higher Received count
+            maxHops: 35
         }
     },
 

plugins/core/delivery-loop.js

@@ -0,0 +1,28 @@
+'use strict';
+
+// Set module title
+module.exports.title = 'DeliveryLoop';
+
+// Initialize the module
+module.exports.init = (app, done) => {
+    const MAX_HOPS = app.config.maxHops || 25;
+
+    app.addHook('message:headers', (envelope, messageInfo, next) => {
+        let receivedLength = envelope.headers.get('Received').length;
+
+        if (receivedLength > MAX_HOPS) {
+            // too many hops
+            app.logger.info('DeliveryLoop', 'Too many hops (%s)! Delivery loop detected for %s, rejecting message', receivedLength, envelope.id);
+            let err = new Error(`A delivery loop was detected which causes this email which caused the email to be undeliverable (${receivedLength})`);
+            err.name = 'SMTPResponse';
+            err.responseCode = 500;
+            return next(err);
+        }
+
+        // allow the message to pass
+        return next();
+    });
+
+    // all set up regarding this plugin
+    done();
+};