Mails are sent by the sendmail.py detector[0], and for detectors there's no way to know if a submission came from the web browser or from a mail.
We could avoid sending a mail to the submitter in both the cases (browser and email), but I prefer to receive them.

[0]: http://svn.python.org/view/tracker/instances/python-dev/detectors/sendmail.py?view=markup