Come on guys, be serious about it. Suggestions like these won't work. Stop assuming the RMT are robots or idiots. Even if this was implemented, it would not put a dent on the RMT. They will just switch to different methods - face to face transactions, or using the marketboard - get the buyer of 10 million gil to put up a fire shard for 10 million gil so they gilsellers pay for it. There are so many other ways than moogle mail.
So be serious about it. We all know there is only one true way to get rid of RMT, which has been proven in JP servers - if there are no gilbuyers, then the gilsellers will cease to exist. Target the gilbuyers, the root of the RMT problem.