It would be nice if the mailbox items had a time stamp! That way you can keep track of when people buffed you, how long it has been since a trade offer was sent to you etc.
Also, it would be great to be able to mark certain mails as "favourites" or "starred" which could then also be found in a separate "mailbox" as shown below.
(It is true, I got MAD MS Paint skillz)
I often find myself writing the same or similar mails with game advice to our newest guild members, and having to go mailbox spelunking to locate the message I want to copy can be time consuming. Such a feature would let me save some mails I copy or use frequently for easy access and save loads of time.
The natural follow up from such an update could be to let users organise mail using their own folders?