Imagine the mass amounts of players. The server would be sending out sooo many messages because they don't all land at the same exact time. Maybe, if the server sent out a mass message to everyone on a specific day (who completed it) at 12am EST?
Also, if the /mail could be time-stamped, that would be awesome. Sent: 9/20/2015 00:00:00 .... Just an idea. I know a lot of people don't clear their mail after reading it to read it again or for future reference....
Good idea, but gets really involved with programming and isn't "worth the time" right now.
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