diff --git a/sparkpost.class.php b/sparkpost.class.php
index bc5e3708..f7b4b6c0 100644
--- a/sparkpost.class.php
+++ b/sparkpost.class.php
@@ -121,7 +121,9 @@ public function init_sp_http_mailer($args)
         return $args;
     }
 
-    static function is_sandbox($email) {
-        return array_slice(explode('@', $email), -1)[0] === 'sparkpostbox.com';
+    static function is_sandbox($email)
+    {
+        $email_splitted = array_slice(explode('@', $email), -1);
+        return $email_splitted[0] === 'sparkpostbox.com';
     }
 }