The problem there is because the avatar/username is in a table row in the same table as the post-content. The avatar/username row has two table cells but the post-content row only has one cell. On Outlook, the avatar cell in the top row is given the full width of the post-content cell from the row underneath it.
The solution is to use a nested table for the avatar/username:
<table>
<tr>
<td>
<table>
<tr>
<td class="user-avatar"></td>
<td class="username"></td>
</tr>
</table>
</td>
</tr>
<tr>
<td class="body"><!-- post content --></td>
</tr>
</table>