Hi,
When I sent an email via the email icon for the invoice, it showing me message “The email has been sent to the customer.”. Which means the email has been sent successfully.
You have to check if your wp_mail() function is working properly or not. I see a notice “Mandrill: wp_mail has been declared by another process or plugin, so you won’t be able to use Mandrill until the problem is solved.” in the admin side which can be a hint for this issue but I am not sure. You have to take the help of a developer to fix this notice which can be from plugin or custom code.
You can check if emails from our plugin or WP site are sent or not via plugin https://wordpress.org/plugins/wp-mail-logging/. Our error message will only display if wp_mail() function returns false.
Regards,
Patrik