this is a deliberate ripp off

Two weeks ago Vodacom send me a notice that my bill has gone to R 16 800. Now R 21 000. Believing this must be a mistake I went into Vodacom and then they confirmed it is correct. They asked to see my phone and showed me that I must have downloaded 11.5 GIG of email data. I was then told to check with my service providers and they confirmed that there is no such a thing. My total emails downloaded for the last three years do not come to 11.5 GIG. This all happened over a period of one month exactly. So Vodacom call me after I sent them a letter and then asked me to look into it. According my setting and there I will see that I definitely used the data. So I look and yes it is showing that there is 11.5 GIG of data used by my email folder. But there are no emails either in my inbox or outbox. So I question this and Vodacom tells me is could be a bouncing email. I question that this is even possible. And why would Vodacom that can see my average bill idefinitelys no more than around R 1000 per month let me continue. My data is always off and I only ever use Wi-Fi. This is very suspicious and something that Vodacom is doing. They are prepared to half my bill but even thats crazy.