Hi there,
the mailpoet CSS is setting the elements width and adding padding, but they haven’t set it to box-sizing: border-box;
You can add this CSS to fix that and stop it from overflowing:
.mailpoet_form_popup {
box-sizing: border-box;
}
Note: there CSS is also removing the background image from the form on smaller screens ( max-width: 500px).
Note: GP 3.0 with the Customizer > General > Structure set to Flexbox will automatically apply border-box to the content elements.