I saw that some people had this annoying problem when upgrading to KitKat on their Xperia devices, as well as on the Xperia Z2. Luckily the workaround is not too difficult to do.
Download Nine (mail app) from the google play store. I am also sure if you use the Stock Kitkat mail app it should also work from here: http://www.androidpolice.com/2013/10...lide-out-menu/
Nine lets you use the PIN exchange security policy instead of the password on the default (Sony) mail app after upgrading to KitKat!
Oh yeah, don't forget to remove the Exchange account from the default mail application and remove the passcode afterwards BEFORE installing Nine!