While I would like to see a mail system, I'd like for it to be with the stipulation that items and gil can't be sent through it. That's just my personal opinion, one that I realize is probably going to be unpopular. But I'm for anything that promotes face time between players, and the current trading system does that.
I think what would be more useful would be to implement a treasury system for companies. Members of the same company have access to a treasurer (basically a company retainer) who has multiple layers of access, including tiers who can be accessed by anyone in the company. Want to give a bunch of stuff to your spouse? Drop it off and give that person access, and viola, the problem is solved.