if($_GET['utm_source']) {
$utm = $_GET['utm_source'];
if($utm === 'yandex') {
//...
} else {
//...
}
}
<FORM ACTION="mailto:nameserver@mail.ru" METHOD="POST" ENCTYPE="text/plain">
<INPUT TYPE="text" name="name" placeholder="Введите имя">
<INPUT TYPE="submit" VALUE="Отправить">
</form>