There are two things I think the mail needs: the ability to cut, copy and paste, and the ability to see outgoing mail.

Editing abilities would be useful for, say, guild officers and masters to paste guild text over without having to retype everything again and again, and also for traders to paste lists of stuff to other people if they should want to do that.

Having an outbox would be useful for seeing who you have responded to, proving that you sent stuff with attachments, and keeping track of attachments you sent.