Delivery Reciept SPAM

It has recently been found that SPAMMERS are using the delivery reciept functionality of most mail clients to find valid email accounts. 

It is reccomended that you switch off this feature to prevent you email account from sending a response back to the SPAMMERS letting them know that your email address is valid.

Further reading: 
http://blog.trendmicro.com/spam-using-e-mail-delivery-notifications-to-verify-valid-addresses/