Mail using extreme amount of data (1 gb an hour)

Hi All.

I have a mid-2012 Macbook Air (13 inch) running OSX 10.8.4.

When I logged on this morning, I noticed a significant amount of data being sucked up through my mobile broadband - in excess of 1gb over an hour. I had no updates running, no downloads going, no streaming etc - basically the Mac was sitting idle with a few small text emails going back and forth.

I killed off every app/software with direct connections to the internet and found that downloads stopped when I closed Mail and Safari. It increased somewhat when I reopened Safari so I killed the cookies, wiped out all the old website data and restarted the app. It seemed to fix the problem, till I reopened Mail and the usage exploded again.

I installed Rubbernet and it shows that mail is using a massive amount of data over a few minutes - screen dump attached.

My email account uses my own domain mapped through gmail. I haven't changed any of the settings in there at all over the last few months.

Any ideass? I'm desperate to stop the data suck!