Outlook sending the same reply every 15 minutes

I replied to an email several hours ago. My sent box shows that outlook has continued to send a reply back to the sender every 15 minutes or sometimes more frequently. I have deleted the original email and all responses and there is no copy of it anywhere that i can see. Yet outlook continues resend it - at least thats what it shows in the sentbox. I have tried to contact the sender to see if they are actually getting all these messages but no response from them.