border-radius in Outlook Classic

Before

<a href="https://example.com"
  style="background-color: #6d28d9; color: #fff;
         padding: 12px 32px; border-radius: 6px;
         text-decoration: none; display: inline-block;">
  Click Here
</a>

After

<!--[if mso]>
<v:roundrect xmlns:v="urn:schemas-microsoft-com:vml"
  href="https://example.com"
  style="height:44px; v-text-anchor:middle; width:200px;"
  arcsize="14%" strokecolor="#6d28d9" fillcolor="#6d28d9">
  <w:anchorlock/>
  <center style="color:#fff; font-family:Arial,sans-serif;
    font-size:14px; font-weight:bold;">Click Here</center>
</v:roundrect>
<![endif]-->
<!--[if !mso]><!-->
<a href="https://example.com"
  style="background-color: #6d28d9; color: #fff;
         padding: 12px 32px; border-radius: 6px;
         text-decoration: none; display: inline-block;">
  Click Here
</a>
<!--<![endif]-->