diff --git a/src/Plugin/Mail/DevelMailLog.php b/src/Plugin/Mail/DevelMailLog.php
index d29ec7f..6f9c81d 100644
--- a/src/Plugin/Mail/DevelMailLog.php
+++ b/src/Plugin/Mail/DevelMailLog.php
@@ -33,6 +33,8 @@ use Exception;
  */
 class DevelMailLog extends PhpMail {

+  const MAX_FILENAME_LENGTH = 255;
+
   public function composeMessage($message) {
     $mimeheaders = array();
     $message['headers']['To'] = $message['to'];
@@ -60,7 +62,8 @@ class DevelMailLog extends PhpMail {
       '%subject' => $message['subject'],
       '%datetime' => date('y-m-d_his'),
     );
-    return $output_directory . '/' . $this->dirify(str_replace(array_keys($tokens), array_values($tokens), $output_file_format));
+    $filename = $this->dirify(str_replace(array_keys($tokens), array_values($tokens), $output_file_format));
+    return $output_directory . '/' . Unicode::substr($filename, 0, self::MAX_FILENAME_LENGTH);
   }

   private function dirify($string) {