Setting is "HTML only", but mail has plain text, too

[SventB]

In the Direct Mail Configuration I've set the "Format of mail content" to "HTML only", but the mail, which was sent by Direct Mail, contains Plain text and HTML:

--_=_swift_v4_1538125251_6ec0686b5bbd858df5ed1edff282734c_=_
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable



--_=_swift_v4_1538125251_6ec0686b5bbd858df5ed1edff282734c_=_
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable

The part of the plain text is empty.

I fear this could lead to problems with some email clients, e.g. some of them show an empty email (when they prefer the plain text mail to the HTML mail).