Manually verify signed mails containing item attachments

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  I am trying to verify the digital signature of emails created with Outlook 2016.

  A signed email has an attachment named "smime.p7m" that usually contains a valid mime message. The signature is the last part and encoded in base64.
  The file is starting like this:

  MIME-Version: 1.0
  Content-Type: multipart/signed;
  	protocol="application/x-pkcs7-signature";
  	micalg=SHA1;
  	boundary="----=_NextPart_000_00E8_01D38BC6.44D87BE0"

  When i send an email containing an item attachment (a calendar item, another email) together with another file outlook will create a winmail.dat attachment and put that into the smime.p7m file. The wrapping mime message however looks like it is in a kind of binary format?

  0�	*�H��
  
  ��0�
  
  10	+��0�	*�H��
  
  
  ��$��Content-Type: multipart/mixed;
  	boundary="----=_NextPart_000_0000_01D38BB7.A3884850"
  X-MS-TNEF-Correlator: 70BDC141AF8BD301
  
  This is a multipart message in MIME format.

  This does not work with the mime parser i use (sun.mail.com).
  If i manually strip the characters up to the Content-Type the mime parser can read the message and extract the winmail.dat attachment, but the signature cannot be parsed, maybe because there are more "wrapping" characters at the end.

  Does anybody know what format Outlook uses for this kind of smime.p7m file?

  Tuesday, January 16, 2018 8:50 AM