I have a very simple html email structure:
<table border="0" cellpadding="0" cellspacing="0" width="100%">
<tr>
<td align="center">
<table border="0" cellpadding="0" cellspacing="0" width="600">
<tr>
<td align="left">
This is where my content goes.
</td>
</tr>
</table>
</td>
</tr>
</table>
I've used this structure for years and it always worked in various email clients because it's so basic.
But in the Outlook Web App it doesn't center the inner table.
It adds style="text-align:center" to the td with align="center". But style="text-align:center" only centers text, no tables. So the inner table aligns left in the OWA.