HOW TO STOP MAIL SENDING EMPTY EMAILS BACK TO SENDERS

I have three email accounts and I think only one is affected. When someone sends me an email to that account, when I click on to the email to open it, it immediately sends back an empty email to the sender. However, it can repeat sending the empty email multiple times. Here is an example (this is an email I sent to myself):

From: Focus on Families

Subject: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re:

Date: 11 July 2014 11:08:12 PM NZST

To: Focus on Families

I really have no idea why this is happening, and it is causing a lot of problems with frustrated clients receiving empty emails from me. At the moment I have over 200 empty emails in my inbox that have originated from a single email that was opened by me.

Any help would be gratefully received - in plain english please - technology is not my strongest point!