diff --git a/js/libtextsecure.js b/js/libtextsecure.js
index eea7ce22..5895e0f5 100644
--- a/js/libtextsecure.js
+++ b/js/libtextsecure.js
@@ -102,7 +102,17 @@
     MessageError.prototype = new ReplayableError();
     MessageError.prototype.constructor = MessageError;
 
+    function UnregisteredUserError(number, httpError) {
+        this.name = 'UnregisteredUserError';
+        this.number = number;
+        this.code = httpError.code;
+        this.message = httpError.message;
+        this.stack = httpError.stack;
+    }
+    UnregisteredUserError.prototype = new Error();
+    UnregisteredUserError.prototype.constructor = UnregisteredUserError;
 
+    window.textsecure.UnregisteredUserError = UnregisteredUserError;
     window.textsecure.SendMessageNetworkError = SendMessageNetworkError;
     window.textsecure.IncomingIdentityKeyError = IncomingIdentityKeyError;
     window.textsecure.OutgoingIdentityKeyError = OutgoingIdentityKeyError;
@@ -38570,9 +38580,13 @@ OutgoingMessage.prototype = {
 
     transmitMessage: function(number, jsonData, timestamp) {
         return this.server.sendMessages(number, jsonData, timestamp).catch(function(e) {
-            if (e.name === 'HTTPError' && (e.code !== 409 && e.code !== 410 && e.code !== 404)) {
+            if (e.name === 'HTTPError' && (e.code !== 409 && e.code !== 410)) {
                 // 409 and 410 should bubble and be handled by doSendMessage
+                // 404 should throw UnregisteredUserError
                 // all other network errors can be retried later.
+                if (e.code === 404) {
+                    throw new textsecure.UnregisteredUserError(number, e);
+                }
                 throw new textsecure.SendMessageNetworkError(number, jsonData, e, timestamp);
             }
             throw e;
diff --git a/libtextsecure/errors.js b/libtextsecure/errors.js
index 27a0eebc..e30f1cad 100644
--- a/libtextsecure/errors.js
+++ b/libtextsecure/errors.js
@@ -101,7 +101,17 @@
     MessageError.prototype = new ReplayableError();
     MessageError.prototype.constructor = MessageError;
 
+    function UnregisteredUserError(number, httpError) {
+        this.name = 'UnregisteredUserError';
+        this.number = number;
+        this.code = httpError.code;
+        this.message = httpError.message;
+        this.stack = httpError.stack;
+    }
+    UnregisteredUserError.prototype = new Error();
+    UnregisteredUserError.prototype.constructor = UnregisteredUserError;
 
+    window.textsecure.UnregisteredUserError = UnregisteredUserError;
     window.textsecure.SendMessageNetworkError = SendMessageNetworkError;
     window.textsecure.IncomingIdentityKeyError = IncomingIdentityKeyError;
     window.textsecure.OutgoingIdentityKeyError = OutgoingIdentityKeyError;
diff --git a/libtextsecure/outgoing_message.js b/libtextsecure/outgoing_message.js
index 316eb299..0444fb12 100644
--- a/libtextsecure/outgoing_message.js
+++ b/libtextsecure/outgoing_message.js
@@ -79,9 +79,13 @@ OutgoingMessage.prototype = {
 
     transmitMessage: function(number, jsonData, timestamp) {
         return this.server.sendMessages(number, jsonData, timestamp).catch(function(e) {
-            if (e.name === 'HTTPError' && (e.code !== 409 && e.code !== 410 && e.code !== 404)) {
+            if (e.name === 'HTTPError' && (e.code !== 409 && e.code !== 410)) {
                 // 409 and 410 should bubble and be handled by doSendMessage
+                // 404 should throw UnregisteredUserError
                 // all other network errors can be retried later.
+                if (e.code === 404) {
+                    throw new textsecure.UnregisteredUserError(number, e);
+                }
                 throw new textsecure.SendMessageNetworkError(number, jsonData, e, timestamp);
             }
             throw e;