app/views/mailers/_button_left.html.erb
<table role="presentation" border="0" cellpadding="0" cellspacing="0" class="btn btn-primary">
<tbody>
<tr>
<td align="left">
<table role="presentation" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td> <%= yield %> </td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>