confirmation email not sending

Hi

This plugin is just outstanding, which meets my requirement.
But I am facing a issue with Confirmation email.
Once user submits the form there it says that “you will get a confirmation email shortly” but which is actually not arriving to user.
Note that I am able to send email user via “Email Your Volunteers” option in backend.

http://preview.enroutedigitallab.com/sune-demo/volunteer-opportunity/test-opportunity/

thanks in advance.