diff --git a/app/Exceptions/AlreadySubscribedException.php b/app/Exceptions/AlreadySubscribedException.php
new file mode 100644
index 00000000..dacf00dc
--- /dev/null
+++ b/app/Exceptions/AlreadySubscribedException.php
@@ -0,0 +1,24 @@
+<?php
+
+/*
+ * This file is part of Cachet.
+ *
+ * (c) Alt Three Services Limited
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace CachetHQ\Cachet\Exceptions;
+
+use Exception;
+
+/**
+ * This is the already subscribed exception class.
+ *
+ * @author Graham Campbell <graham@alt-three.com>
+ */
+class AlreadySubscribedException extends Exception implements ExceptionInterface
+{
+    //
+}
diff --git a/app/Exceptions/ExceptionInterface.php b/app/Exceptions/ExceptionInterface.php
new file mode 100644
index 00000000..15ce0eee
--- /dev/null
+++ b/app/Exceptions/ExceptionInterface.php
@@ -0,0 +1,22 @@
+<?php
+
+/*
+ * This file is part of Cachet.
+ *
+ * (c) Alt Three Services Limited
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace CachetHQ\Cachet\Exceptions;
+
+/**
+ * This is the exception interface.
+ *
+ * @author Graham Campbell <graham@alt-three.com>
+ */
+interface ExceptionInterface
+{
+    //
+}
diff --git a/app/Exceptions/Transformers/ModelNotFoundTransformer.php b/app/Exceptions/Transformers/ExceptionTransformer.php
similarity index 65%
rename from app/Exceptions/Transformers/ModelNotFoundTransformer.php
rename to app/Exceptions/Transformers/ExceptionTransformer.php
index 69ea37ba..2e4fb2b9 100644
--- a/app/Exceptions/Transformers/ModelNotFoundTransformer.php
+++ b/app/Exceptions/Transformers/ExceptionTransformer.php
@@ -11,17 +11,19 @@
 
 namespace CachetHQ\Cachet\Exceptions\Transformers;
 
+use CachetHQ\Cachet\Exceptions\ExceptionInterface;
 use Exception;
 use GrahamCampbell\Exceptions\Transformers\TransformerInterface;
 use Illuminate\Database\Eloquent\ModelNotFoundException;
+use Symfony\Component\HttpKernel\Exception\BadRequestHttpException;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 
 /**
- * This is the model not found transformer class.
+ * This is the exception transformer class.
  *
  * @author Graham Campbell <graham@alt-three.com>
  */
-class ModelNotFoundTransformer implements TransformerInterface
+class ExceptionTransformer implements TransformerInterface
 {
     /**
      * Transform the provided exception.
@@ -32,8 +34,10 @@ class ModelNotFoundTransformer implements TransformerInterface
      */
     public function transform(Exception $exception)
     {
-        if ($exception instanceof ModelNotFoundException) {
-            $exception = new NotFoundHttpException('Resource not found');
+        if ($exception instanceof ExceptionInterface) {
+            $exception = new BadRequestHttpException($exception->getMessage());
+        } elseif ($exception instanceof ModelNotFoundException) {
+            $exception = new NotFoundHttpException('Resource not found.');
         }
 
         return $exception;
diff --git a/app/Handlers/Commands/Subscriber/SubscribeSubscriberCommandHandler.php b/app/Handlers/Commands/Subscriber/SubscribeSubscriberCommandHandler.php
index e96fb8fe..9ab617ed 100644
--- a/app/Handlers/Commands/Subscriber/SubscribeSubscriberCommandHandler.php
+++ b/app/Handlers/Commands/Subscriber/SubscribeSubscriberCommandHandler.php
@@ -14,6 +14,7 @@ namespace CachetHQ\Cachet\Handlers\Commands\Subscriber;
 use CachetHQ\Cachet\Commands\Subscriber\SubscribeSubscriberCommand;
 use CachetHQ\Cachet\Commands\Subscriber\VerifySubscriberCommand;
 use CachetHQ\Cachet\Events\Subscriber\SubscriberHasSubscribedEvent;
+use CachetHQ\Cachet\Exceptions\AlreadySubscribedException;
 use CachetHQ\Cachet\Models\Subscriber;
 use Illuminate\Foundation\Bus\DispatchesJobs;
 
@@ -26,10 +27,16 @@ class SubscribeSubscriberCommandHandler
      *
      * @param \CachetHQ\Cachet\Commands\Subscriber\SubscribeSubscriberCommand $command
      *
+     * @throws \CachetHQ\Cachet\Exceptions\AlreadySubscribedException
+     *
      * @return \CachetHQ\Cachet\Models\Subscriber
      */
     public function handle(SubscribeSubscriberCommand $command)
     {
+        if (Subscriber::where('email', $command->email)->first()) {
+            throw new AlreadySubscribedException("Cannot subscribe {$command->email} because they're already subscribed.");
+        }
+
         $subscriber = Subscriber::create(['email' => $command->email]);
 
         if ($command->verified) {
diff --git a/config/exceptions.php b/config/exceptions.php
index 06dcaffe..11064dd6 100644
--- a/config/exceptions.php
+++ b/config/exceptions.php
@@ -27,7 +27,7 @@ return [
 
     'transformers' => [
         'GrahamCampbell\Exceptions\Transformers\CsrfTransformer',
-        'CachetHQ\Cachet\Exceptions\Transformers\ModelNotFoundTransformer',
+        'CachetHQ\Cachet\Exceptions\Transformers\ExceptionTransformer',
     ],
 
     /*