Thanks. I'll take a look asap.

IMP 6.2.5.

Changes have been made in Git (master):

commit 584b744c9d759089d37c8a13e1b9bc8f831e3f77
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Dec 30 10:27:50 2014 -0700

     [mms] Fix parsing broken S/MIME messages that don't include the 
smime-type content-type parameter (Request #13661).

     Conflicts:
             imp/docs/CHANGES
             imp/package.xml

  imp/docs/CHANGES              |    2 +
  imp/lib/Mime/Viewer/Smime.php |   51 +++++++++++++++++++++++++++++++++++-----
  imp/package.xml               |    5 ++++
  3 files changed, 51 insertions(+), 7 deletions(-)

http://github.com/horde/horde/commit/584b744c9d759089d37c8a13e1b9bc8f831e3f77

Changes have been made in Git (FRAMEWORK_5_2):

commit 3d07c17bd47c33368c939525a13f7a1dacc20b5a
Author: Michael M Slusarz <slusarz@horde.org>
Date:   Tue Dec 30 10:27:50 2014 -0700

     [mms] Fix parsing broken S/MIME messages that don't include the 
smime-type content-type parameter (Request #13661).

  imp/docs/CHANGES              |    2 +
  imp/lib/Mime/Viewer/Smime.php |   51 +++++++++++++++++++++++++++++++++++-----
  imp/package.xml               |    6 +++++
  3 files changed, 52 insertions(+), 7 deletions(-)

http://github.com/horde/horde/commit/3d07c17bd47c33368c939525a13f7a1dacc20b5a

Although I agree with you and it would be nice if all clients were 
model citizens and followed the latest version of the specs to the 
letter, the media-type is deprecated, but not illegal afaik, it's like 
using SHA1 instead of sha-1 for the micalg parameter.

RFC 5751 [3.2] is saying SHOULD not MUST

Also, RFC 5751 [5.1] contains this
Type name: application
Subtype Name: pkcs7-mime
Required Parameters: NONE
Optional Parameters: smime-type/signed-data
                      smime-type/enveloped-data
                      smime-type/compressed-data
                      smime-type/certs-only
                      name

I don't know if this encoding only comes from one family of clients of 
which very few use Horde or if it's a larger problem.

"properly encoded" is very questionable.

First, it's using the long deprecated MIME type.

Second, it does not contain the smime-data parameter.  From RFC 5751 [3.2]:

    Because there are several types of application/pkcs7-mime objects, a
    sending agent SHOULD do as much as possible to help a receiving agent
    know about the contents of the object without forcing the receiving
    agent to decode the ASN.1 for the object.  The Content-Type header
    field of all application/pkcs7-mime objects SHOULD include the
    optional "smime-type" parameter, as described in the following
    sections.

In other words... that is some garbage input.

Some additional information.

The 2nd part can't be decrypted, because it's a signed message, but 
Horde treats it as another encrypted message
Trying to decrypt it will throw an error
34374513640:error:21072071:PKCS7 routines:PKCS7_decrypt:wrong content 
type:pk7_smime.c:546:

The Smime lib should test the 2nd part using `-cmsout` before using it.

cmsout will contain something like this
Content-Type: application/pkcs7-mime; smime-type=signed-data; name="smime.p7m"

Missing attachment

I have a message which is properly encoded and which can be decrypted 
via openssl cms, but wich Horde does not understand.

Horde can decrypt the first part:
smime.p7m (6 KB)
The data in this part has been encrypted via S/MIME.

But does not understand the decrypted message:
  X-pkcs7-mime (4 KB)
The data in this part has been encrypted via S/MIME.
Could not decrypt S/MIME data.

The 2nd part looks like this
Content-Type: application/x-pkcs7-mime
Content-Transfer-Encoding: base64

MIAGCSqGSIb3DQEHAqCAMIACAQExDTALBglghkgBZQMEAgEwgAYJKoZIhvcNAQcB
oIAkgASCA4NDb250ZW50LVR5cGU6IG11bHRpcGFydC9hbHRlcm5hdGl2ZTsgYm91
...
DI2fdyAMfp2GSTTIVdpgOeDt6J+32QG0ppzfeOv8EGCvd7r7BO6gqy3XS54m7XNV
SGllRWATSg7MKOkAAAAAAAA=

I'm attaching that part for testing