根据MSDN，EmailComposeTask的Body属性不支持HTML Content。所以你的html内容不起作用。

As per MSDN, Body property of EmailComposeTask doesn't support HTML Content. So your html content will not work.

http://msdn.microsoft.com/En-Us/library/windows/apps/microsoft.phone.tasks.emailcomposetask.body

您可以使用第三方SDK发送带有html内容的电子邮件

You can use 3rd Party SDK to send email with html content