[Plugin: Jetpack by WordPress.com] Contact form not sending email

The contact form works fine when I set it up without an email address to send to, i.e. when it uses my default email address.

But when I change the destination email address in the email notifications tab, the contact form stops working. What I mean is, I can go and fill out the contact form on the web page, press submit, but I don’t get the usual message showing the email has been sent, it just returns me to the top of the form. So there is no confirmation message, nothing in my feedback area, no email.

As I said, this all works fine when I leave that email address blank, then I get the confirmation message and email fine.