Email signature zoom on iOS Mail

I'm running into some bizarre formatting issues with iOS Mail. I've made a fairly simple HTML email signature 500px wide which renders fine in browsers, desktop clients, Android and iOS Gmail, but when receiving it on native iOS mail it overflows the screen and the user has to scroll horizontally to see the whole thing, rather than the viewport scaling down to fit (which is what all the other clients do). Now this is where it gets weirder. If I put a jpg underneath that's 200px high, it scales down and fits everything in. Halve the jpg height, and it goes off-screen again. So it appears the viewport width is directly proportionate to the like, height of the signature... I don't suppose anyone's come across this before and knows if there's anything I can do, short of sticking a huge jpg underneath? I've included a jsFiddle of my HTML (cheers arkascha) - hope it helps? jsfiddle.net/nn56d3wr