linear-gradient in Outlook Classic

Before

<td style="background: linear-gradient(135deg, #667eea, #764ba2);
            padding: 40px; color: #fff;">
  Content here
</td>

After

<!--[if gte mso 9]>
<v:rect xmlns:v="urn:schemas-microsoft-com:vml" fill="true"
  stroke="false" style="width:600px;">
  <v:fill type="gradient" color="#667eea" color2="#764ba2"
    angle="135" />
  <v:textbox inset="0,0,0,0">
<![endif]-->
<td style="background-color: #667eea;
            background: linear-gradient(135deg, #667eea, #764ba2);
            padding: 40px; color: #fff;">
  Content here
</td>
<!--[if gte mso 9]>
  </v:textbox>
</v:rect>
<![endif]-->