"Please enter valid e-mail address" when subscribing

Ghost seems not to accept any e-mail addresses “subscribing” to the newsletter (not a member account), giving a general “please enter valid address error”. Could you help? Thank you.

https://tuxphones.com, Ghost 3.0.2

I tried Subscribe on your site and (as seen in the screenshot below) upon submitting I did receive a confirmation message on the web page, but did not receive the promised confirmation email: