CSS properties which add visual decorations but are not supported can be replaced with image tag.
border-radius is not supported in Yahoo! Mail, Outlook 2007/10/13 +, Outlook 03/Express/Mail & Android 4 (Gmail) +
To work around this we can add images with border radius in them i.e buttons with left & right side as image:
<td><img src="img-border-left.png"><td> <td style="background: #000; color: #fff">Button Text</td> <td><img src="img-border-right.png"><td>
Here is a list of property that should be avoided and used as image instead: