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What is a smime.p7s File and How to Open It?

Did you receive an email with a smime.p7s file attachment? Do you have questions such as what does it mean, is it safe and should I open it or not? If you see this smime.p7s file attachment in your email, you need not worry. It’s used to confirm the sender’s identity and it allows to exchange encrypted emails.

What is a S/MIME.p7s File?

Secure/Multipurpose Internet Mail Extensions (S/MIME) is one of the IETF standards used for public-key encryption and signing MIME data. It offers cryptographic security functionalities like authentication, message integrity and non-repudiation of origin, data encryption and privacy for email applications.

However, before anyone can make use of S/MIME in an email application, an Email and Document Signing Certificate has to be obtained from a publicly recognized CA, such as Comodo. And, once the Email Signing certificate file is purchased and installed on the user’s computer, they will be able to send the digitally signed emails with a digital signature, which comes in the SMIME.p7s file format.

If your email client such as Outlook has installed an Email Signing certificate and has signed the outgoing message, it will automatically send a smime.p7s file along with the outgoing message.

For example, in outlook, an email signed with a trusted certificate can be found by clicking on the red ribbon.

Furthermore, if you click on that red ribbon or any other icon, which indicates the sender of the email is verified, you will see the sender’s validated information along with who issued the Email signing certificate. For example:

Other Email Hosts

However, questions do arise if the signed messages arrive in other mail hosts, in which the SMIME.p7s file is seen as an attachment. For example:
Here, the message is displayed in the Gmail browser interface and not in Outlook. As it’s not supporting, .p7s is found as an attachment. However, the signature is processed and the red ribbon is shown, as Outlook does and the message is safe:

How to Open an SMIME.p7s File?

Yes, technically, you can open the .p7s file using Microsoft Outlook, but it’s not something that is for the users. It’s generally used by email clients like Outlook to authenticate the sender of the email message. So, there’s nothing much you will get by opening it. Again, below are the steps you can follow, if you want to open the P7S file.

  • On your computer, open the folder which contains the P7S file.
  • Double click the file.
  • From the list of default applications, select “Microsoft Outlook 2010” and click “Open.”

Who Needs Email Signing Certificate?

Email & Document Signing Certificates, also known as CPACs (Comodo Personal Authentication Certificates), are used for emails as well as specific documents. Signing helps in many ways such as:

  • Verifying the identity of the sender of the email to know that the message is authentic
  • Alerting you if the document or the email is tampered and it can even restrict access
  • Finally, it helps in encrypting an email message so that only the intended recipient can read it


SMIME is the standard used to encrypt an email. Email clients support it and recognize smime.p7s that comes as an attachment and verify the contents.

Nevertheless, sometimes it happens that you may find the smime.p7s file as an attachment, especially if you’re using other email hosts such as Gmail. If that’s the case, then no need to worry, it’s completely safe to trust that email. It simply means that email host which you’re using doesn’t support .p7s file like Outlook and it’s displayed as an attachment.

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