diff --git a/src/Fetch/Server.php b/src/Fetch/Server.php
index 1ddbb9c..e366b3d 100644
--- a/src/Fetch/Server.php
+++ b/src/Fetch/Server.php
@@ -163,7 +163,7 @@ public function setAuthentication($username, $password)
     public function setMailBox($mailbox = '')
     {
         if (!$this->hasMailBox($mailbox)) {
-            return false;
+            throw new \RuntimeException('No mailbox found');
         }
 
         $this->mailbox = $mailbox;