One minor fix: the content area didn’t contain any padding. I copied this bit of style from the emails that I receive from meta:
<td class="with-dir" style="border-collapse:collapse!important;color:#0a0a0a;line-height:1.3;margin:0;padding:16px;vertical-align:top;word-wrap:normal;text-align:left">
%{email_content}
</td>