Files with the .P7M extension contain encrypted message contents and potential attachments in accordance with the PKCS#7, which stands for Public Key Cryptography Standards no. 7. The files contain S/MIME type data, which stands for Secure/Multipurpose Internet Mail Extensions, a secure version of MIME.

Sending messages in this format prevents unauthorized access to them, as opening such messages usually requires entering a password – namely a private key that matches the public key assigned to the message. This format is, for example, used to transfer sensitive medical or financial information.