Only drawback i can see is the amount of extra mail that's generated. This would take a large amount of extra room on the servers. Which is why the prob brought the chat feature out in the first place
I think its a sharp idea.... should be fairly easy to implement, won't effect game balance and is essentially glutton-free. So it has that going for it.
This is an excellent idea, and is akin to mailing lists as used in many other areas of the interweb.
The forwarding of multiple messages is severely flawed, and far too cumbersome right now, and this idea should be relatively easy to implement.
To simplify it, may I suggest that the recipient lists are used, but with a reply all option? There's already a suggestion to expand the number of recipient lists, and this would go hand in hand quite nicely.