Auto sending e-mails

I received a spam e-mail which was a phishing scam, but having came from a contact I thought it fine and opened it. When I realised what it really was I deleted it and all the other spam ones I have received since have been successfully 'swept'. The problem is that my sent box is filling up at a ridiculous rate with the same spam e-mail forwarding itself hundreds of times - in 4 days it had sent from my sent box over 600 times. Deleting them does nothing and they keep sending from the sent box.

Can someone advise me on what to do, I've scanned my computer and it has came back with no virus / malware found.

Thanks