I agree with Tom on this one, the sendmail.php should be straight forward, your variables in sendmail should relate to your element input names in your html form, so for example;
< ?php
$country = $_POST['country'] ; //get the country
$email = $_POST['email'] ; //get the returned email
// do this for all elements you require
//i would recommend having a $message variable and use it
//to contain all the info you want sent, then pass that to the sendmail
// rather than having lots of variables passed to the mail function e.g.
$message = " From: $name \n
Country: $country \n
etc";
mail("example@example.com", $subject, $message, $from);
?>
The thank you and validation are pretty straight forward, i would recommend using if mx exist for email to make sure users provide a real email address and not just one that passes your validation structure for a valid email, there is plenty of documentation on how to do it out there.