I came up with the idea why they don't implement this system... yet.
Implement this mail system need another "system" to send/receive/store mails that circulate in system..I must say it's a big load.
And from my point of view, they "Didn't" design this system at first place because they didn't plan to implement AH at first place too. ( as you can see in other MMO, the item you buy/bid from AH, you can get it via mail system. )

So..No design, then no mail system, then no server to operate this task.

Why they buy the time with some excuse like server issue?
Answer is simple... YOU PLAYING THIS GAME FOR FREE!

Why SE have to invest in Server/System and get more cost for the game that can't generate income yet.

Conclusion;
I think SE added this feature in their fix list but.. set it to very low priority :P