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: <firstname.lastname@example.org> X-MS-Exchange-Organization-AuthSource: server2.example.com X-MS-Exchange-Organization-AuthAs: Internal X-MS-Exchange-Organization-AuthMechanism: 04 X-Originating-IP: [184.108.40.206]
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).