I'm trying to understand the S/MIME implementation, and maybe I'm wrong. I'm so stuck that I will try to explain ... Sign a mail works as I thought it would work. But, I supposed that when you send a ciphered email, you're using the Public Key of receiver, who has the Private key for decryption. So I supposed that outlook would use the certificate that you introduce on "Contact - Certificates".

But I see that the cipher's certificate, using Office 2007 and Office 365, it's always the same: the sender certificate.

I mean, I send a ciphered mail to a contact, for who I have a certificate configured on Outlook. But the receiver can not open it because it was ciphered with my certificate, not his (In fact I can open it on his computer, plugin my smart card on his card reader).

¿Could someone give me an advice about this?

2 Answers 2


Well, finally I got the issue. I was using smart card certificate, but and I think its a 'certificate purposes' issue. I have got 2 new S/MIME certificate (from free provider ACTALIS), and it works as I expected.

The receiver only need the public key of my certificate in order to decrypt mails


As I know, when using digital signatures, the sender will sign the message with his own certificate private key and the recipient uses the sender's public key to compare the email signatures and verify the sender's identity. So, please try to check if you have enabled digital signatures feature and use Digital signature and email encryption to work together. I did some research and found this article: Understanding S/MIME which might be useful to you, for your reference.

  • Mail signing works as you say, but I had troubles with ciphering. Thanks Sep 3, 2020 at 7:12

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