Anyone have any good tips for up to date complete contact forms with validation and everything that is easy to implement?

ATM my form looks like this, no validation.

Code HTML4Strict:
<form id="contact-form" action="email.php" method="post">
	<div class="input-wrapper1">
		<label for="name">namn</label><input type="text" name="name" id="name" />
	</div>
	<div class="input-wrapper2">
		<label for="mail">epost</label><input  type="text" name="mail" id="mail" />
	</div>
	<label for="msg">ärende</label><textarea name="msg" id="msg"></textarea>
	<button type="submit" name="send" id="send"><i <i class="icon-angle-circled-right"></i></button>
</form>

Code PHP:
<?php 
 
$name = $_POST['name'];
$mail = $_POST['mail'];
$msg = $_POST['msg'];
$formcontent="From: $name \n Mail: $mail \n Message: $msg";
$recipient = "mail@outlook.com";
$subject = "Contactform submit from RB";
$mailheader = "From: $mail \r\n";
mail($recipient, $subject, $formcontent, $mailheader) or die("Error!");
echo "Thank You!";
 
?>