Changes have been made in Git (develop):

commit d9f98a4b9feddf1f89c1f2439295563a1d8a453a
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Wed Mar 28 01:40:03 2012 -0600

     Bug #11058: Fix bad method call

  framework/Mime/lib/Horde/Mime/Part.php |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/d9f98a4b9feddf1f89c1f2439295563a1d8a453a

I can confirm this working, too.

Yeah, my fault.  I switched the method of sanity checking halfway 
through and forgot to change this back.

Reopen this ticket if you see any further issues.

Changes have been made in Git (master):

commit d9f98a4b9feddf1f89c1f2439295563a1d8a453a
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Wed Mar 28 01:40:03 2012 -0600

     Bug #11058: Fix bad method call

  framework/Mime/lib/Horde/Mime/Part.php |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/d9f98a4b9feddf1f89c1f2439295563a1d8a453a

"array_key_exists($opts['encode'])" should be 
"array_key_exists('encode',$opts)"

beside this it seems ok

Changes have been made in Git (develop):

commit 054d6c36fd451c0711c2eb45a1653f0b7e48c018
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Mar 27 23:56:19 2012 -0600

     Bug #11058: Encrypted messages will be base64 encoded, so ok to 
send 8bit/binary, even if signed.

  imp/lib/Compose.php |    8 ++------
  1 files changed, 2 insertions(+), 6 deletions(-)

http://git.horde.org/horde-git/-/commit/054d6c36fd451c0711c2eb45a1653f0b7e48c018

Changes have been made in Git (develop):

commit 53f124c40670b6a1b25b7bf06fe9c904bbe1e263
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Mar 27 23:48:02 2012 -0600

     [mms] Ensure that PGP & S/MIME signed message bodies are not 
altered after the signature is calculated (Bug #11058).

  imp/docs/CHANGES    |    2 ++
  imp/lib/Compose.php |   18 +++++++++++++++---
  imp/package.xml     |    4 +++-
  3 files changed, 20 insertions(+), 4 deletions(-)

http://git.horde.org/horde-git/-/commit/53f124c40670b6a1b25b7bf06fe9c904bbe1e263

Changes have been made in Git (develop):

commit 86d6dec3184a2bd470f165f5aebbf9751fefee58
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Mar 27 23:43:16 2012 -0600

     [mms] Add option to allow encoding options to be defined for 
Horde_Mime_Part#send() (Bug #11058).

  framework/Mime/lib/Horde/Mime/Part.php |   40 
++++++++++++++++++++------------
  framework/Mime/package.xml             |   12 ++++----
  2 files changed, 31 insertions(+), 21 deletions(-)

http://git.horde.org/horde-git/-/commit/86d6dec3184a2bd470f165f5aebbf9751fefee58

Changes have been made in Git (master):

commit 054d6c36fd451c0711c2eb45a1653f0b7e48c018
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Mar 27 23:56:19 2012 -0600

     Bug #11058: Encrypted messages will be base64 encoded, so ok to 
send 8bit/binary, even if signed.

  imp/lib/Compose.php |    8 ++------
  1 files changed, 2 insertions(+), 6 deletions(-)

http://git.horde.org/horde-git/-/commit/054d6c36fd451c0711c2eb45a1653f0b7e48c018

Do these commits fix things?

Changes have been made in Git (master):

commit 53f124c40670b6a1b25b7bf06fe9c904bbe1e263
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Mar 27 23:48:02 2012 -0600

     [mms] Ensure that PGP & S/MIME signed message bodies are not 
altered after the signature is calculated (Bug #11058).

  imp/docs/CHANGES    |    2 ++
  imp/lib/Compose.php |   18 +++++++++++++++---
  imp/package.xml     |    4 +++-
  3 files changed, 20 insertions(+), 4 deletions(-)

http://git.horde.org/horde-git/-/commit/53f124c40670b6a1b25b7bf06fe9c904bbe1e263

Changes have been made in Git (master):

commit 86d6dec3184a2bd470f165f5aebbf9751fefee58
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Mar 27 23:43:16 2012 -0600

     [mms] Add option to allow encoding options to be defined for 
Horde_Mime_Part#send() (Bug #11058).

  framework/Mime/lib/Horde/Mime/Part.php |   40 
++++++++++++++++++++------------
  framework/Mime/package.xml             |   12 ++++----
  2 files changed, 31 insertions(+), 21 deletions(-)

http://git.horde.org/horde-git/-/commit/86d6dec3184a2bd470f165f5aebbf9751fefee58

Thank you, after your last comment,i set 'encode' => self::ENCODE_7BIT;
in the following toString call as a workarround, which is essentially 
the same (changes a single line only :-))

Until this is fixed you can use the attached workaround.

The problem is not in postfix but in Horde/Mime/Part.php, function 
send(), starting on line 1669.

There is code that detects if MTA supports 8BITMIME (RFC 1652) or 
BINARYMIME (RFC 3030) extensions and re-encodes the mimePart.

A S/MIME part cannot be reencoded from quoted-printable to anything 
else because it breaks the signature.

One of possible solutions would be to extend the send function by 
giving it a new boolean parameter to use only 7bit encoding 
(quoted-printable) for S/MIME signed messages and set this parameter 
on sign time.

Hi Michael,
it's not so simple. I test this internally, so only one MTA(postfix) 
is involved.
I'm also not sure, that this header is the reason for the verification 
failure.
However, if it is added by the MTA, shouldn't it be added by IMP 
before signing?
I've attached 3 complete Messages.
mail-1 contains only the letter 'a' (and my signature) and can be verified.
mail-2 contains only the letter 'ä' (and my signature) and cannot be verified.
mail-3 is what imp appended to my Sent-folder(when sending mail-2), 
this one can be verified.

We don't add this.  Sounds like a MTA is adding this somewhere in transit.

S/MIME signed messages cannot be verified if body contains 8-bit character.

It seems, that a 'Content-Transfer-Encoding: 8bit' header is added 
after signing.