exim4 vs gmail FROM field
gmail_login:
       driver = plaintext
       public_name = LOGIN
       client_send = : myaccount1@gmail.com : mypassword

$to      = 'myaccount3@gmail.com';
$subject = 'the subject';
$message = 'hello';
$headers = 'From: myaccount2@gmail.com' . "rn" .
    'Reply-To: myaccount2@gmail.com' . "rn" .
    'X-Mailer: PHP/' . phpversion();

if (mail($to, $subject, $message, $headers)) {
  echo 'mail() Success!' . "<br />n";
}
else {
  echo 'mail() Failure!' . "<br />n";
}