<form method="get" action="test.php">
  <input type="checkbox" name="check[0]" value="mail_01@mail.at" id="mail_01">
  <label for="mail_01">mail_01</label>
  <br />
  <input type="checkbox" name="check[1]" value="mail_02@mail.at" id="mail_02">
  <label for="mail_02">mail_02</label>
  <br />
  <input type="checkbox" name="check[2]" value="mail_03@mail.at" id="mail_03@mail.at">
  <label for="mail_03@mail.at">mail_03</label>
  <br />
  <input type="checkbox" name="check[3]" value="mail_04@mail.at" id="mail_04">
  <label for="mail_04">mail_04</label>
  <br />
  <input type="checkbox" name="check[4]" value="mail_05@mail.at" id="mail_05">
  <label for="mail_05">mail_05</label>
  <br />
  <input type="checkbox" name="check[5]" value="mail_06@mail.at" id="mail_06">
  <label for="mail_06">mail_06</label>
  <br />
  <input type="checkbox" name="check[6]" value="mail_07@mail.at" id="mail_07">
  <label for="mail_07">Smail_07</label>
  <br />
  <input type="checkbox" name="check[7]" value="mail_08@mail.at" id="mail_08">
  <label for="mail_08">mail_08</label>
  <br />
  <input type="hidden" value="1" name="s">
  </form>
<?php
if ($_GET['s']=="1") {
$subject="Anfrage";
$link_title="e-mail Programm &ouml;ffnen";


$mail_adressen=implode(",",$check);
echo "$mail_adressen";
echo "<br />";
echo "<a href=\"mailto:$mail_adressen?subject=$subject\">$link_title</a>";
}
?>