use a while() loop w/ the database query, and put sleep(5) in it,
somthing like,
<?php
$query = "SELECT * FROM table";
$result = mysql_query($query) or die(mysql_error());
while ($row = mysql_fetch_array($result)) {
mail($row['email'], 'subject', 'message');
sleep(5);
}
?>
I understand your reasoning for this, but there's a better way to send bulk emails.. digging around PEAR & their email whatever-it's-called might be worth the Google.