421 resources temporarily unavailable

I have an email stuck in my outbox that is constantly being sent (over 80 times, every time I turn my computer on). I cannot delete it or stop it being sent. A message comes up occasionally saying 421 resources temporarily unavailable. Can you assist?

 

I occasionally get this message with Outlook 2007 and have no idea what it means. My message does get sent, though, despite the error..

I have an email stuck in my outbox that is constantly being sent (over 80 times, every time I turn my computer on). I cannot delete it or stop it being sent. A message comes up occasionally saying 421 resources temporarily unavailable. Can you assist?