WOW! I just went to confirm my way of sorting for you & instead of being 95% full my mailbox is 1% !!! They've matched Yahoo with 250MB storage!
Maybe it's got something to do with that changeover, but what I was going to tell you no longer works - hopefully it'll be temporary. If it gets put back, what you can do is click on the heading of the Date column & the order toggles between ascending & descending.
As I say, I hope that function returns, as it would be such a pain having to work through to the end of 250MB of stored enails!
H