<form method="post" action="mail.php">
<input type="text" name="nome" required>
<input type="text" name="cognome" required>
<input type="text" name="email" required>
<input type="submit" value="Invia">
</form>
//mail.php
<?php
$n = $_POST["nome"];
$c = $_POST["cognome"];
$e = $_POST["email"];
$a= $n.$c.$e;
mail("info@4i.it", "dati utenti" , $a);
?>