SMIME Mails from Outlook 2010 can't get decrypted on Mac Mail or Entourage

HannoKirchhoff
HannoKirchhoff used Ask the Experts™
on
Today we dicovered a problem with encrypted Mails.
Sending with Outlook 2007 to Mac Mail oder Entourage works well, but if you use a Outlook 2010 Client for encrypting a mail, this mail can't be decrypted on a Mac. In the other direction it works all well, so Mac can send encrypted to Outlook 2010 that can decrypt the mail.

Anyone any idea?

p.s. of course we tried diferent PCs and Macs
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Commented:
In the the header of the Mails I could only find one difference:
the one who can be decrypted correctly (send from Outlook 2007)
 Content-Type:       application/x-pkcs7-mime; smime-type=enveloped-data; name="smime.p7m"
 Content-Disposition:       attachment; filename="smime.p7m"
 Content-Transfer-Encoding:       base64
 Mime-Version:       1.0
the one what is noc decrypted (send with Outlook 2010):
 Content-Type:       application/pkcs7-mime; smime-type=enveloped-data; name="smime.p7m"
 Content-Disposition:       attachment; filename="smime.p7m"
 Content-Transfer-Encoding:       base64
 Mime-Version:       1.0

So Content rype differs, on Outlook 2010it is not:   application/x-pkcs7-mime

Commented:
pkcs7-mime is correct, but x-pkcs7-mime has been around longer. It is possible that pkcs7-mime is not being recognized by Mail.

Easy to test: locate one of the affected emlx files (in a messages folder a few layers deep in ~/Library/Mail/), edit the header with textwrangler or another text editor (to add the"x-"), and see mail can read it now.
Microsoft confirmed the error in Outlook 2010 and gave us an patch theat worked:

Ich möchte Sie bitten, auf dem sendenden Outlook 2010 Client folgenden Registry Key zu setzen:
HKCU\Software\Microsoft\Office\14.0\Outlook\Security
Fügen Sie einen REG_DWORD hinzu, nennen Sie diesen “UseIssuerSerialNumber” und setzen Sie den Wert auf 1.

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