The database is not involved at all, I asked for the default email charset.
But the message is problematic anyway, so this is probably an issue 
with your system. This is an unencoded 8bit message, and not all mail 
servers, libraries, etc. work fine with those. That's why they are 
actually not allowed.

No, in Admin - Configuration - Horde - Database is $conf[sql][charset] 
set to utf-8.
I'm using Horde Groupware Webmail Edition  1.2.3, Horde (horde)   
3.3.6, Webmail (imp)  H3 (4.3.6)
on a virtual Debian-Server with Apache/2.0.54 (Debian GNU/Linux) DAV/2 
PHP/5.2.0-8+etch9~bpo31+1 mod_ssl/2.0.54 OpenSSL/0.9.7e

Please have a look on the appended screen-shots: mail1.jpg shows the 
original message with correct umlauts, the mail2.jpg is the 
answer-window with some utf-8 (?) signs. I wonder, why there is a 
difference between the original and the answer...
I tried this with some client-pcs (Linux, Windows, Firefox, MS-IE) - 
the result is always the same: original msg is OK, quoted answer is 
wrong...
May I should give You login-datas for a test-mailbox, so you can test 
it by yourself?
Thanks again for Your answers!

This mail works perfectly fine for me. All characters are correct in 
the reply window. I guess you use the German locale and have 
iso-8859-1 as your default email charset? There is no conversion 
happening in this case, because the original message is iso-8859-1 too.

Oh, sorry - here my next trial...

A pasted message doesn't help. Please *upload* the *raw* message.

Thanks for YOur answer!!
Here is the original source code - domain names are changed to "xxx":

Return-path: <>
Envelope-to: michael.kloeren@xxx.de
Received: from smtprelay04.ispgateway.de ([80.67.31.31])
        by zuse.all-connect.net with esmtp
        (Exim 4.63)
        (envelope-from <spuling@xxx.de>)
        id 1ORNUy-0006n4-1i
        for michael.kloeren@xxx.de; Wed, 23 Jun 2010 12:48:10 +0200
Received: from [84.172.165.216] (helo=a3)
        by smtprelay04.ispgateway.de with esmtpa (Exim 4.68)
        (envelope-from <spuling@xxx.de>)
        id 1ORNZa-0006mO-9h; Wed, 23 Jun 2010 12:52:50 +0200
Date: Wed, 23 Jun 2010 12:53:08 +0200
From: spuling@xxx.de
To: Undisclosed recipients:;
Organization: VWA Rhein-Neckar e.V.
X-Mailer: Blat v2.6.2 w/GSS encryption, a Win32 SMTP/NNTP mailer 
http://www.blat.net
Message-ID: <01cb12c2$Blat.v2.6.2$454ec85c$608bb8234a4@xxx.de>
Subject: =?ISO-8859-1?Q?Unternehmensf=FChrung_II_und_III?=
Content-Transfer-Encoding: 8BIT
Content-Type: text/plain; charset=ISO-8859-1
X-Df-Sender: spuling@xxx.de
X-Spam-Score: 1.6 (+)
X-Spam-Score-Integer: 16
X-Spam-Report: Das all-connect SDM System (auf Server "zuse.all-connect.net")
        hat fuer diese E-Mail hinsichtlich der Spam-Wahrscheinlichkeit eine
        Bewertung vorgenommen. Bei Fragen wenden Sie sich bitte an das
        all-connect Service Team: service@all-connect.net
        Vorschau auf E-Mail:  Sehr geehrte Studierende des 6. Semesters, die 
Vorlesung
        und Übung in Unternehmensführung III am 02.07. und 09.07. sowie die
        Übung Unternehmensführung II am 12.07. findet in der VWA Geschäftsstelle
        statt. [...]
        1.6 Gesamtpunkzahl auf folgender Grundlage:
        Pkt. Regel                  Beschreibung (en)"
        -----------------------------------------------------------------------
        0.6 NO_REAL_NAME           From: does not include a real name
        1.2 RCVD_IN_BHFTSG         RBL: Received via a blackhole in
        blackholes.five-ten-sg.com
        [80.67.31.31 listed in blackholes.five-ten-sg.com]
        -0.1 AWL                    AWL: From: address is in the auto white-list

Sehr geehrte Studierende des 6. Semesters,

die Vorlesung und &#65533;bung in Unternehmensf&#65533;hrung III am 
02.07. und 09.07. sowie die &#65533;bung Unternehmensf&#65533;hrung II 
am 12.07. findet in der VWA Gesch&#65533;ftsstelle statt.

Bitte teilen Sie uns unbedingt mit, wenn Sie die CP-Klausur 
Unternehmensf&#65533;hrung II und III am 17.07.2010 schreiben wollen.

Beste Gr&#65533;&#65533;e

Margitta Bauer

Please upload an example message.

I received an email with german umlauts (from X-Mailer: Blat, 
http://blat.net). When I open this mail, umlauts are displayed 
correctly.
But when I click to "answer" the quoted text contains no umlauts, but 
many wrong coded signs.