#11 Content-Type text/html not set

[3i5fuch5]

I can't set the Content-Type of the mail to text/html.

IsBodyHtml = true

and

var nvc = new NameValueCollection {{"Content-Type", "text/html"}};
Headers = { nvc }

are ignored by the lib. The Content-Type of the Mail is always:
Content-Type: multipart/mixed; boundary="32d5b61c-4617-4b20-aac7-06fa4f4ebbc5"