When sending an e-mail in the standard OS X Mail application a swoosh tone will be played. This tone cannot be disabled through the standard mail settings. With the following defaults write command you can disable the sound. Quit the Mail application before you use the command.
defaults write com.apple.mail MailSentSoundPath dummy
Restart the mail application, when sending a new message the send-mail tone will not be played.
Restore the ‘send-mail’ tone with:
defaults delete com.apple.mail MailSentSoundPath