I found what is causing this rendering bug in emails sent from the server.
When you insert a layout table with the following column sizes : 255px 20px 255px here is what is inserted in the xsl :
<table>
<colgroup>
<col style="WIDTH: 255px" /><col style="WIDTH: 20px" /><col style="WIDTH: 255px" />
</colgroup>
<tbody>
<tr><td></td><td></td><td></td></tr>
</tbody>
</table>
Now when the view is sent by mail, here is what I receive :
<table>
<colgroup><col style="WIDTH: 255px" /><col style="WIDTH: 20px" /><col style="WIDTH: 255px" /></colgroup>
<tbody>
<tr><td width="20"></td><td width="255"></td><td></td></tr>
</tbody>
</table>
A workaround is to manually edit the xsl file and use the style attribute of <td> to set the width :
<tbody>
<tr><td style="WIDTH: 255px"></td><td style="WIDTH: 20px"></td><td style="WIDTH: 255px"></td></tr>
</tbody>
I still don't understand why Forms services implementation has this behaviour, if someone has an idea or another workaround that don't require to edit the xsl please let me know.