Basically,
There should be a way to opt in to allowing received in-server mail to cc/ my personal registered email. Since SMTP forwarding is free, this is a added benefit to players with zero risk to the server host.
There should also be a means for a player to access all mail from all servers in a central location, either within each server a selection to check "other server mail" or all mail goes to a single mail system with server tabs - similar to if not identical to the in-server forum options.
This idea has been killed, shot down, stomped on, rolled over and backed up and canned by every developer in control of the game since go live. Likely due to the non-revenue aspect.
Would be nice to see it implemented, since you know, PR stuff...