Accented characters (umlauts) disappear in email text

Hi there. In the email sent by Contact Form to me and the subscriber all special characters like the german “umlauts” äöüßÄÖÜ or french accented characters just disappear – instead of “Straße” we get “Strae”, from the message body as well as from the input data. The form is here: http://hanfjournal.de/abo/

Though the email header clearly says it’s UTF-8, the text in the source code of the mail has no characters instead of special characters, not even wrong transcriptions.

Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: 8bit

I have searched the forums for this kind of problem all say it’s about the UTF-8 setting. WordPress nowadays is always in UTF-8, there is not even a way to change to ISO or something else like in older WordPress versions.

I also checked the current internal PHP character encoding of the website with this code:

<?php
echo mb_internal_encoding();
?>

…it says UTF-8.

I added as additional header in Contact Form settings:

Content-Type: text/plain; charset=UTF-8

It did not help either.

I can only use special characters in the email body by activating HTML content at the response mail editor and using HTML entities, but still special characters from the form data are all missing.

https://wordpress.org/plugins/contact-form-7/