I have a small idea for 1.5, due to the release of hoppers, mailboxes could be availbe as in dropping items or using minecarts with chests, but maybe it's more efficient to add a /cwg command that will make a fully public chest, like, an accsessible chest even on the wg, something like /copen (open as in open for all), also the possibility to make /cmodify to override the wg restrictions as in letting someone out of the wg acsess the chest he has been /cmodified, I'm not sure if it's either a suggestion or a bug, but as for the mail system, it could be possible using somekind of "/copen" to make a box, and below it make an hopper to another box and such, thus creating a working mail box
The mailbox system can be done with minecarts with chests, but they can be easialy destroyed by random grieffers, thus the possibility of "/copen" could benefit much more, also it can be for donation box(if the donation is for the public that is).
The mailbox system can be a wonderfull way to give gifts, and i recommand adding a letter using a writteble book.