Hehe, last time this way brought up was many years ago but I think it was combined with a request for a banking system. One down one to go.

It's not likely that it will happen soon but if we got banks then it is likely they will eventually get around to a mail system. Mind you we are looking at like 3 years. But a player system would be better. It's just hard to coordinate and would require a great deal of effort and organization which I don't think most players would want to do.
A sample system for this would be to have a group of players hire themselves out as couriers, preferably one with access to a guildhouse, a chest, ect so that there is common storage between them. Then they can "check in" packages and letters that need to be delivered and "check out" those that have been delivered. Regardless of what tracking system is used, there is no way around it, you have to make sure it is accessible to all the couriers and that it keeps track of who for sure has received their deliveries and who hasn't. Just doing the RP will get messy otherwise.
You'll need to have a fair amount of people contributing to the effort though. There are timezone issues, people not being on all the time, players being busy or far away, etc. It would be an undertaking to say the least.