If you see a P7s file extension in your email, someone's sent you a digitally signed document. When someone sends you an important document, the digital signature confirms the sender's identity and the document's authenticity. The P7s extension shows up if your email program can't handle digital signatures.
Using digital signatures requires both public and private encryption keys. The person sending you the email creates a hash -- a mathematical summary -- of the contents and encrypts it with his private key. When you receive the email, you make a second hash, then decrypt the sender's hash with your public key. If the two hashes match, the document is authentic and hasn't been tampered with.
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To open a P7s you need an email client that works with Public Key Cryptography Standard No. 7. These programs include Thunderbird, Outlook, Apple Mail and PostBox. If your email client doesn't handle PKCS 7, you see the smime.P7s attachment. The attachment only contains the hash so you don't need to open it to read the document. However, you might want to check with the sender to verify what he sent.