There are so many reasons that disabling the outgoing mail sound in OS X might be a good idea for some. To disable the sound, write a key called MailSentSoundPath with the contents of a path to the new sound into the com.apple.mail defaults domain (in this case we’re just not gonna’ send it any real data so it doesn’t play a sound):
defaults write com.apple.mail MailSentSoundPath /devnull
The close Mail and open it again. Try sending yourself an email and the sound should be gone. To enable the sound again, just delete the MailSentSoundPath key:
defaults delete com.apple.mail MailSentSoundPath
