send.php

 <?php

  $to = $_POST['email'];
  $from = "送信元メールアドレス";
  $subject = $_POST['subject'];
  $body = $_POST['body'];

  $url = 'https://api.sendgrid.com/';
  $user = 'SendGrid username';
  $pass = 'SendGrid Password';

  $params = array(
    'api_user'  => $user,
    'api_key'   => $pass,
    'to'        => $to,
    'subject'   => $subject,
    'text'      => $body,
    'from'      => $from,
  );

  $request =  $url.'api/mail.send.json';

  $session = curl_init($request);
  curl_setopt ($session, CURLOPT_POST, true);
  curl_setopt ($session, CURLOPT_POSTFIELDS, $params);
  curl_setopt($session, CURLOPT_HEADER, false);
  curl_setopt($session, CURLOPT_SSLVERSION, CURL_SSLVERSION_TLSv1_2);
  curl_setopt($session, CURLOPT_RETURNTRANSFER, true);

  $response = curl_exec($session);
  curl_close($session);

  print_r($response);

 ?>