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Someone sends an email with (among others) the following headers

Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
X-MS-Has-Attach: yes
X-MS-Exchange-Organization-SCL: -1
X-MS-TNEF-Correlator: <guid@server1.example.com>
X-MS-Exchange-Organization-AuthSource: server2.example.com
X-MS-Exchange-Organization-AuthAs: Internal
X-MS-Exchange-Organization-AuthMechanism: 04
X-Originating-IP: [1.2.3.4]

to my Microsoft Exchange account, and the mail is somehow signed (and not encrypted) by a certificate which is issued by a CA within our organization.

In Outlook on Windows, I can read this email just fine.

However, clicking the 'Decode message' button which my the standard Android email client shows me (on Android 4.1.2, through the Exchange connection) results in a message 'Cannot show encoded messages. Certificate not installed.' (Label and message actually show up in Dutch on my phone, I've guess-translated them back to English.)

I have a .p7b-file containing the CA's public key (and I can get it in X.509 DER encoded or Base-64 encoded format as well).

My question: How can I make the Android email client trust this CA certificate, so that it allows me to read these mails?

Update: I'm not asking for how to add a certificate with a private key (as can be used for authenticating to a wi-fi network), but a certificate with just a public key (which can be use to verify signatures).

  • Have you tried this answer: android.stackexchange.com/a/16730/5342 – geffchang Feb 18 '14 at 13:39
  • @geffchang Thanks for the hint, but no success. I tried this just now, and (1) I can install the certificate successfully; but (2) I suddenly need to protect my phone using a pattern, since this apparently tries to use the certificate for signing in on e.g. WiFi (so as a 'client certificate', as if it had a private key, which it doesn't); however (3) the certificate does show up in Settings -> Security -> Trusted sign-in information -> User; but (4) reading the signed email still does not work. – MarnixKlooster ReinstateMonica Feb 18 '14 at 14:00

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