linear-gradient in HEY Mail

Before

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

After

<!-- Always declare a solid background-color before the gradient.
     Clients that strip gradients will show the fallback color. -->
<td style="background-color: #667eea;
            background: linear-gradient(135deg, #667eea, #764ba2);
            padding: 40px; color: #fff;">
  Content here
</td>