Some email clients (notably Windows Desktop Outlook) collapse tables with no content, even if there are dimensions or padding.
To prevent these clients from collapsing the spacer table, you can add an invisible that serves as content. Then zero out the character's font-size and line-height so it doesn't impact the <table>s dimensions. The height (or width) attribute is now the only thing affecting the spacer's dimensions.
font-size
line-height
<table>
<table border="0" cellpadding="0" cellspacing="0" role="presentation"> <tr> <td height="40" style="font-size: 0; line-height: 0;"> </td> </tr> </table>
It was common practice to use a 1x1 transparent .gif as a spacer, but this method is deprecated.
.gif