It's absolutely obvious their server architecture is an complete mess. That's the only imaginable reason adding trivial features like mailboxes take so much time.

I don't really remember them bragging about their architecture, but if they ever did, that's probably one of the most ridiculous things that happened during the game development. On par with them saying they will compete with Wow.

And I would pay to have an complete description of their server architecture, this would be so much interesting.