Unable to send e-mail on checkout simulation

Hello, when I make order with "Register account" and complete the checkout with "Simulate checkout completion" on admin/commerce/orders/*/edit, I get error saying "Unable to send e-mail. Contact the site administrator if the problem persists."

What could be the problem?

I'm using commerce checkout login 7.x-2.0-beta1, and commerce 7.x-1.13.