app/views/mailer/_spacer_table.html.erb
<table <%= placeholder_table_styles %>>
<tr>
<%= placeholder_cell('20px', vertical: false) %>
</tr>
</table>
<table <%= placeholder_table_styles(width:'100%',style: 'width:100%;max-width:700px;') %>>
<tr>
<%= placeholder_cell('12px', vertical: true) %>
<td>
<%= yield %>
<table>
<tr>
<%= placeholder_cell('40px', vertical: false) %>
</tr>
</table>
</td>
<%= placeholder_cell('12px', vertical: true) %>
</tr>
</table>