In order to have a popup displayed after each mail was successfully submitted 
you'll need to do the following steps:

Please coply all the files from the archive in the directory 
PATH_TO_WEBMAIL_DIR/webmail/default/private/standard directory.

If you want to change the text that will be displayed in the popup please edit the
file page_mail_sent_dictionary.hsp and replace the text in the double quotes.

Restart the server, and the popup should appear.
