I have instaled WP Mail Log and WP SMTP. By checking WP Mail Log I have seen the email sent, but without the e-mail text field. When I have enabled
WP SMTP the problem goes further. It only sends mail when you are on the computer. When responsive design changes the layout to mobile, the WP Mail Log does not show any e-mails and WP SMTP does not send them.

You can test one demo contact @ nikaandgrega.si/test – Observe the coloring of the fields when hitting send. On the mobile device, the e-mail field will not color green, but the rest will.