While the web and email are built on the same technologies (HTML and CSS), email applications don’t adhere to the same standards as web browsers. Each email application has its own rendering engine which determines what code is supported and how emails are displayed. 
The bad news for HTML professionals are that all of those rendering engines support different HTML tags and CSS properties.


Outlook (Windows) does not support valign - This WORKAROUND will work:


...
<td colspan="3" valign="middle" width="300" height="2" style="width: 300px;font-size: 2px;line-height: 2px;"><hr color="888888" size="2" width="100%"></td>
...

 

Here's the result in Outlook 2016 on Windows: