German Umlauts (ä,ö,ü,ß) corrupt in HTML Mails sent with Outlook 2013 and OXtender 2 for OpenXchange

Today i noticed that HTML Mails I sent from Outlook 2013 looked like this on the receiver side:

�ffentliche �bereinstimmung m��te sein

The german umlauts (special characters)  were somehow misinterpreted as non-printable characters.
 

The original text was:

"Öffentliche Übereinstimmung müßte sein"

How-I-fixed-it:

Looking at the raw mail source content i recognized, there was a lot of css-stuff in mail which seemed to be added by Outlook automatically.
osoft-com:office:office" xmlns:w=3D"urn:schemas-microsoft-com:office:word" =
xmlns:m=3D"http://schemas.microsoft.com/office/2004/12/omml" xmlns=3D"http:=
//www.w3.org/TR/REC-html40">ft Word 15 (filtered medium)">
link=3D"#954F72">

=EF=BF=
=BDffentliche =EF=BF=BDbereinstimmung m=EF=BF=BD=EF=BF=BDte sein=

-->
I found out, that there's an option in Outlook to tell it to use CSS for formatting or not.

I disabled this option and my HTML mails look OK now.