I have some frustrations with some mail features and I see I'm not alone on some of them.
1) Why automatically delete in 3 days? maybe someone sends me text of information I need, don't want to forget. Maybe I read something, but have to go out of town for the weekend. Maybe you need to remember their name, or their crafting order, or whatever. There are many reasons why someone may want an empty email of just words kept around for more than three days.
2) Why no REPLY button? Trying to respond to someone (especially not in guild or friend-list, like if you do business), is pretty annoying. Something simple like responding to an email shouldn't require a pen and paper by the computer to do.
These two items really confuse me as I would personally consider them easy to program and pretty natural to have in a game mail system.