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What is a smime.p7s email attachment?

” I get emails with smime.p7s attachments.   What are they?   How do I open one?    Should I be worried? “

Firstly, don’t worry if you see a smime.p7s email attachment.    It’s nothing to worry about and it’s not something you can open.

Smime.p7s is a digital signature which can be sent with an email to confirm the identity of the sender and allow encrypted emails to be exchanged.

Outlook will automatically send smime.p7s files with emails if you’ve installed a digital certificate and signed the outgoing message.   Check Options | Sign when writing a message.

Most people don’t use email digital signatures, which is a pity but understandable given how clumsy Microsoft makes it to use them.

If Outlook and some other email programs detect a smime.p7s  they will automatically process it and show the message as signed.  For example, here’s a message sent to a Gmail address which is linked with Outlook (via IMAP).

Click on the red ribbon to see details of the signature; in effect the contents of the smime.p7s file:

If you have a digital signature yourself, combine that with the other person’s signature to encrypt messages between you.

Other mail hosts

The confusion comes when signed messages arrive in mail hosts which don’t support digital signatures.    All the receiver sees is a smime.p7s attachment.

Here’s the same message as shown above, but this time displayed in the Gmail browser interface instead of Outlook.

Because Gmail in a browser doesn’t support digital signatures, all it shows is the smime.p7s attachment.

Compare that with Outlook.com in a browser which does process the signature and shows a red ribbon, just like Outlook for Windows or Mac.

S/MIME is a standard for email identity and encryption so there’s no good reason for email clients not to support it.  At the very least recognize smime.p7s attachments and verify the contents.

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