This code produces buttons with rounded corners in Outlook 2010 using the Outlook conditional comments () and VML:
<div>
<!--[if mso]>
<v:roundrect xmlns:v="urn:schemas-microsoft-com:vml" xmlns:w="urn:schemas-microsoft-com:office:word" href="http://www.EXAMPLE.com/" style="height:40px;v-text-anchor:middle;width:300px;" arcsize="10%" stroke="f" fillcolor="#d62828">
<w:anchorlock/>
<center style="color:#ffffff;font-family:sans-serif;font-size:16px;font-weight:bold;">
Button Text Here!
</center>
</v:roundrect>
<![endif]-->
<!--[if !mso]> <!-->
<table cellspacing="0" cellpadding="0"> <tr>
<td align="center" width="300" height="40" bgcolor="#d62828" style="-webkit-border-radius: 5px; -moz-border-radius: 5px; border-radius: 5px; color: #ffffff; display: block;">
<a href="http://www.EXAMPLE.com/" style="color: #ffffff; font-size:16px; font-weight: bold; font-family:sans-serif; text-decoration: none; line-height:40px; width:100%; display:inline-block">
Button Text Here!
</a>
</td>
</tr> </table>
<!-- <![endif]-->
</div>
Tested in Outlook 2010 and major browsers only, not OWA, Outlook.com or any mobile browsers. It’s worth a closer look. Credit where due — http://www.industrydive.com/blog/how-to-make-html-email-buttons-that-rock/