[hylafax]: the umlauts thing

Hi all,

I can see from the internet and the archives, that this is a topic with 
hylafax: the character encoding.
Even so, this has been handled in some threads in the mailing list, I 
was not able to find some sort of final solution guide.
only "high level" describtions about ISO/UDF Encoding in postfix.

So here I start again ;-)

My setup:

Client:
Horde Webmailer and Thunderbird "Fat"-Client in most current stable versions

Server:
dovecot-core:
   Installed: 1:2.2.13-12~deb8u4

postfix:
   Installed: 2.11.3-1+deb8u2

hylafax-server:
   Installed: 3:6.0.6-6

The systemwide setting is:

LANG=en_US.UTF-8

I test this for the impacted users by:

 >sudo -u [username] env|grep LANG

regardless which user I take (postfix, uucp, dovenull, dovecot, vmail, 
www-data) I get the correct setting

 From this point I think, the settings are correct.

Now I send a Mail like this to a faxnumber (the examble has been created 
using the Horde Webinterface, but the result is the same, if Thunderbird 
is used:

".....
To:     1234567890@fax
Subject:     ÄÄÄÄÜÜÜÜÖÖÖÖÖääääööööüüüüüßßßßßß
User-Agent:     Horde Application Framework 5
Content-Type:     text/plain; charset=utf-8; format=flowed; DelSp=Yes
MIME-Version:     1.0
Content-Disposition:     inline
Content-Transfer-Encoding:     quoted-printable

Per WebMailer Horde erstellt

ääääää
ÄÄÄÄÄÄ
öööööö
ÖÖÖÖÖÖ
üüüüüü
ÜÜÜÜÜÜ
Ganz viele Umlaute
ßßßßßßß
...."

The received document looks like this (linked picture): 
https://drive.google.com/open?id=1z7IiOQXjyKpYn86gwYIJbKL0RSzQH5yy
Just for clearification: If I send the mail to any mail receipient, all 
characters are translated correct.

Questions:
1) What is the root cause of that?
2) What do I need to tweak, to get it right?
3) Is some documentation about this and the setup in the wild, that I 
somehow missed?

br
Carsten


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