Mail is three days old and won't update?
How to retrieve mail that has been sent, but is not appearing on iPad?
iPad Air
Try to force your iPad to reboot, see if that clears things up.
Hold down the sleep and home keys for about 20 seconds. When you see the silver apple, let go and let it reboot and see if your mail will retrieve.
Beyond that, are you on a normal connection? I know sometimes when I use hotspots my yahoo mail will not retrieve. It's simply something in the hotspot router that wont' allow the connection. In those cases I simply use safari to get my mail until I get on a connection that works. (Sometimes there is no fix, you just need a work around, especially with public hotspots)