e mail data problem

I was shocked to receive my bill of R2800 on my red advantage contract this month. R1800 being for out of bundle data. I used to have an S4 and recently upgraded to an S5, after speaking to the call centre and the lady explaining to me that the S5 will use more data than my old S4, Due to the operating system etc. I am doing nothing different to what I have always done, I have wi fi at home and always connect. I went into my usage and it showed most of my data is from e mails, 2gb e mails in april, which is ridiculous. I have a vodamail e mail address, and my Samsung tablet gets the same amount of e mails. then it reminded me that my phone always shows that I have 493 unread e mails, but it is e mails that I have read from weeks ago. the lady says it is a problem with the phone or I must delete them from the server. why should I , it worked fine on my S4 before, and I haven't changed any settings, as I got the phone out the box I used it, as I said my Samsung tablet works fine, when I read an e mail it doesn't come back a few days later to show I haven't read it. Please sort this out, I would like a credit for all the e mail data for the past few months since I have had this phone.