Try this:
<?php mysql_data_seek($rs_users, 0); $row_rs_users = mysql_fetch_assoc($rs_users); ?>
<?php do { echo(($row_rs_users['email'] != NULL && $row_rs_users['email'] != "")?$row_rs_users['email'].";":""); ?>
<?php } while ($row_rs_users = mysql_fetch_assoc($rs_users)); ?>