Why are my emails being marked as read automatically?

I recently changed from iPhone 4 to 5; I never experienced any email issues with the iphone4. Now my emails on iphone5 ;same account etc) are sometimes marked as read before I view them. This happens every night over night without fail, and invariable happens during the day too (although not always- I can't yet see any pattern in this). It's extremely frustrating as I could rely on looking at my lock screen on iphone4 to see my recent unread emails; and I now cannot rely on this (and there's normally nothing there as its all marked as read in my inbox, before I've seen it! Then I have to check through each message to make sure I haven't missed something).

There are no other devices running to access this inbox; it is set up on my home laptop through outlook (as it was with iphone4); but this is only switched on about once a week just to download the messages.

Please does anyone have a solution to this very frustrating issue? I did not expect that by upgrading I would be worse off!