Hi, how can I open smime.p7m attachments ?

I think you are missing the point. We are not shown an smime.p7m file unless something in Mail.app has failed. If it was just a matter of certificates we should see a message to that point, "This message is not for you" or something. As with others attempting to open this attachment tries to add a certificate to my key chain.

Last night I received two emails which fail to render/decode in both Mavericks and Mountain Lion versions of Mail.app. Send from "X-Mailer: Apple Mail (2.1510)" which the sender full well knows my certificate, if these were encrypted.

I took today some hours to find a solution. Hardest case I had within the last years.

Problem:
Instead of a decrypted mail, shown as plain text, a mail with an attachment called .p7m appears.

What happens technical:

The sender sends an encrypted mail using a s-mime certificate to increase security cause encrypted mails do not contain plain text. Instead a code is used. A public and private key is within this mail.

The receivers mailclient is using these keys to open the mail and decrypt the content.

Why an attachment instead of an Text?
I think there are many reasons a attachment insetad of an decrypted mail is showing:
Here is my opinion:
Original Mail was send by Person 1 from PC1 to Person 2 with PC 2. Person 2 is answering.
Now Person 1 is using a new PC (1a) or different Mail Client. The new Software/PC from Person 1 is not able to decrypt the answer from Person 2 anymore cause the Password/Certificate has changed.

Solution:

Not known at the moment

Workarround:
Use a mail decryption software or plugin for your Browser.

Lets find out a little more. What is the case you had problems with?
Please let me know if this helps you a little bit.

You can't. They're encrypted. Unless you have the key that was used to encrypt them, they're unopenable.

No. The sender wraps the email with your key. If you don't have S/MIME set up, there's no way to open a message someone else sends you.

Maybe tell the person on the other end to not send encrypted emails.

I have a variation on this question.

I SENT a message to someone but after sending the message, I can't seem to read the message myself. My sent message is in my Sent folder. I click on the smime.p7m file and it says: "Do you want to add the certificate(s) from the file "smime.p7m" to a keychain?

If I click on "View Certificates" it says: Unable to display this certificate. The data does not appear to be a valid certificate.

When I click on "Details", the panel Shrinks to so small it is absurd and shows only Unable to display this certificate.

Is this problem with my Certificate? or with the recieving party's certificate?

I see the same behavior on both of my machines where this e-mail is available: I bcc'ed it to myself.

Any idea what is wrong?

It seems that Terry was reporting the same problem I have right now. And he is not missing the point:

What if you properly set up S/MIME, send properly encrypted e-mails to some one who is able to decrypt and read them, but anyway in your own Sent folder you just find the attachment "smime.p7m".

David thinks that something in Mail.app has failed that. I have the same feeling. But what to do now?

I tried already exporting, deleting and re-importing all key and certificates in my key chain, but without a change. Any recommendations (I am using OS X 10.10.1 and Mail 8.1 (1993).

Doesn't look like much has changed here since. Running Mail.app Version 9.1 (3096.5) on El Capitan, I experience the exact same thing.

Properly set up GPGTools work well with OpenPGP, so I have set it to automatically encrypt, when a key is available. There is no setting for just doing it with OpenPGP but leave S/MIME alone, so it also encrypts when I answer an S/MIME-signed Mail. Unfortunately, I cannot read what I wrote after I hit send, until another answer to it comes in and quotes my mail.

It's a shame how this mail client has dilapidated. It used to be a very good program prior to this "make everything more iOSish".

I just encountered an smime.p7m attachment in mail.app (first time in 5 years of mac use). OSX 10.10.5 Yosemite, iMac.

Maybe something to do with the fact that I just renewed some digital certificates at work.

Either way, I forwarded the original (unreadable) e-mail to my gmail account - I could then read the original e-mail, and open all attachments from mail.app (without having to look my gmail, or even leave mail.app).

This is because you can only add a p12 file. Not a .p7m