I have been send a lot of gold seller spam, They always name like spasdsf or asdasfsf or something like that, so i think some solution like this
-Only who add friend with you can send normal mail to you.
-Guild members can send mail to each other.
So if friend want to send each other some item you can C.O.D with some silvers as tax or trade face-by-face.
How do you think about it ?