Дополнительный пробел вызван этой пустой строкой:
<tr>
<td> </td>
</tr>
Попробуйте заменить эту строку на:
<tr>
<td height="0" style="padding: 0; margin: 0; height: 0;"></td>
</tr>
В качестве альтернативы, иногда при работе с Outlook проще всего использовать <br> для пробелов.
Вот ваш полный HTML с добавлением 2 <br> в конце последнего абзаца:
<html>
<body>
<style>
tr {border:none;}
</style>
<table id="container" border="0" background-color="#D3E2E9" width="620px" cellpadding="0" cellspacing="0" style="background:#D3E2E9;">
<tr><td>
<table id="container_border" cellpadding="0" cellspacing="0" style="border:1px solid #85898B; width:570px; margin-left:20px; margin-top:20px; margin-bottom:20px;">
<tr><td>
<!-- content -->
<table id="content" cellpadding="0" cellspacing="0" style="margin-left:15px; margin-top:20px; margin-bottom:20px;padding:10px; width:517px; background:white;">
<tr><td>
<tr>
<td><img src="https://origin.ih.constantcontact.com/fs089/1103957675139/img/7.jpg" style="display:block"></td>
</tr>
<tr>
<td style="font-family:arial,sans-serif;font-size:11px; line-height:150%; color:#000; margin-top:20px; padding:0 52px; font-weight:bold;">Announcing, da, da, da, DAAAAAAAAAAA, the brand new awesomely helpful,
wildly interesting Back Bay Shutter Company website. Full of juicy information
on all of our products, from shutters, shades and blinds to Shoji panels, this
site will help make the job of choosing just the right window treatment a snap.</td>
</tr>
<tr>
<td><img src="https://origin.ih.constantcontact.com/fs089/1103957675139/img/8.jpg" style="display:block"></td>
</tr>
<tr>
<td style="font-family:arial,sans-serif;font-size:11px; line-height:150%; color:#000; margin-top:20px; padding:0 52px; font-weight:bold;">We even have product videos, so you can actually see how a product looks and
works in people's homes (and who doesn't like a peak at someone else's home?).
We also have a blog, brimming with design news and views.<br><br>
</td>
<tr>
<td style="font-family:arial,sans-serif;font-size:11px; line-height:150%; color:#000; margin-top:90px; padding:0 22px; font-weight:bold;">
<table>
<tr>
<td>
<a href="" title="Join Our Facebook Page"><img src="https://origin.ih.constantcontact.com/fs089/1103957675139/img/6.jpg" border="0" style="display:block"></a>
</td>
<td style="font-family:arial,sans-serif;font-size:11px; line-height:150%; color:#000; font-weight:bold;">
So become a Facebook fan and stay updated on what we're doing. And stop by often. We can't wait to show you around.
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td><img src="https://origin.ih.constantcontact.com/fs089/1103957675139/img/3.jpg"></td>
</td></tr>
</table>
</td></tr>
</table>
</td></tr>
</table>
</body>
</html>