The forum devs have stated on multiple occasions that the mail box feature is something that is coming but not in the present/near future because it's not a priority compared to fixing the core gameplay. SE's logic is that mailboxes are necessary but not crucial because their function is simply a long distance trade. The other part of it is the standard "need to fix the server engine first" crap we normally get as an answer for 50% of our questions.

I don't agree with their logic, but that's their answer. Same as the linkshell stuff that has been asked countless times.