This looks like a PHP question, not a SQL question! Anyway, move the <table> out of the loop, and then just style your <tr> tag with a border-top or border-bottom.
<td>
<table class='commenttable'>
<?php
include '../Scripts/connection.php';
$sql = "SELECT id, name, email, subject, comment FROM comment";
$result = $connect->query($sql);
if ($result->num_rows > 0) {
// output data of each row
while($row = $result->fetch_assoc()) {
echo "
<tr style=\"border-bottom: 1px solid #000;\">
<td class='id'>
<u>Loyal Client ". $row["id"] . "</u>:
</td>
<td class='subject'>
" . $row["subject"]. "
</td>
</tr>
<tr>
<td></td>
<td class='comment' colspan='2'>
" . $row["comment"]. "
</td>
</tr>
";
}
} else {
echo " <tr>
<td>
No Comments
</td>
<tr>
";
}
?>
</table>
</td>
