diff --git a/src/Mailgun/Connection/RestClient.php b/src/Mailgun/Connection/RestClient.php
index 63f160b..1c18d12 100644
--- a/src/Mailgun/Connection/RestClient.php
+++ b/src/Mailgun/Connection/RestClient.php
@@ -96,7 +96,7 @@ class RestClient
                 $builder->addResource($file['name'], $file['contents'], $file);
             }
             $body = $builder->build();
-            $headers['Content-Type'] = 'multipart/form-data; boundary='.$builder->getBoundary();
+            $headers['Content-Type'] = 'multipart/form-data; boundary="'.$builder->getBoundary().'"';
         } elseif (is_array($body)) {
             $body = http_build_query($body);
             $headers['Content-Type'] = 'application/x-www-form-urlencoded';
diff --git a/src/Mailgun/RequestBuilder.php b/src/Mailgun/RequestBuilder.php
index f5d5cf9..6df9a12 100644
--- a/src/Mailgun/RequestBuilder.php
+++ b/src/Mailgun/RequestBuilder.php
@@ -65,7 +65,7 @@ class RequestBuilder
         $multipartStream = $builder->build();
         $boundary = $builder->getBoundary();
 
-        $headers['Content-Type'] = 'multipart/form-data; boundary='.$boundary;
+        $headers['Content-Type'] = 'multipart/form-data; boundary="'.$boundary.'"';
 
         return $this->getRequestFactory()->createRequest($method, $uri, $headers, $multipartStream);
     }