<form id="form1" name="form1" method="post" action="yazitemizle.php">
  <label>
  <textarea name="yazi" cols="50" rows="10"></textarea>
  </label>
  <label>
  <input type="submit" name="buton" value="gönder" />
  </label>
</form>

<?php
    if(isset($_POST["yazi"]))
    {
    $c = $_POST["yazi"];
    $ayir = split("@hotmail.com", $c);
    foreach ($ayir as $a) {
    if (!empty($a)) {
    echo "$a";
    }
    }
    }
?>