vos NO queres que salga los salto de lineas?
$mensaje = '
<table>
<tr>
<td colspan="2">
<img src="'.PATH_URL.'/img/imagen.png" />
</td>
</tr>
<tr>
<td>Name</td>
<td>'.$nom.'</td>
</tr>
<tr>
<td>E-mail</td>
<td>'.$email.'</td>
</tr>
<tr>
<td>Telefon</td>
<td>'.$tel.'</td>
</tr>
<tr>
<td>Firma</td>
<td>'.$fir.'</td>
</tr>
<tr>
<td>Kommentar</td>
<td>'.str_replace("\n"," ",str_replace("\r"," ",$comment)).'</td>
</tr>
</table>
';
Fijate ahi!
Abrazo