O.K. I tried it, and it doesn't work. I believe that the "mail sent" sound has to be in the systems folder to begin with, and it can't be added by a mail client. For example: The "sounds" settings in my wife's iPhone have a new mail, and a sent mail category. You can attach ringtones for each. The SGSIII doesn't have this format(in stock, anyway), and i probably would have to root, in order to add the "mail sent", to the sounds file. If i'm wrong, and i hope that i am, please let me know. Thanks for any help.
P.S. After using a MacBook Pro, and an iPhone for so long, i just got used to hearing that sound when i sent out an email or a text msg. I guess that it's a "comfort" thing.
I wouldn't go with stock email app, esp considering I keep changing ROMs every now and then. Having a non stock email app takes your dependence on the app that's bundled with the ROM every time you change it. Plus stock email is kinda very low on features compared to many free emails apps out there.