File extension P7M files are an implementation of the Secure/Multipurpose Internet Mail Extension (S/MIME) standard which supports public key encryption. S/MIME provides essentially two cryptographic services which can be used by E-Mail applications. The first is digital signatures, which can be used to authenticate a message, check its integrity and prove its origin or source. The second is encryption, which is used to ensure that data remains both private and secure, even if an E-Mail is intercepted by a third-party.

As files with File extension P7M are encrypted, it is not possible to extract the contents without the appropriate software and decryption key. E-Mail applications with S/MIME support can be configured to automatically retrieve the necessary key to decrypt received P7M files and display the resultant message as an ordinary E-Mail. However, clients without support, such as web-based E-Mail services, will simply show the encrypted P7M E-Mail as an attachment.

Encryption is utilised by P7M files to ensure data privacy even if the E-Mail was accidentally sent to the wrong address or was intercepted during transmission. However, this has the disadvantage that anti-virus software is unable to detect malware, as it will not be able to decrypt the E-Mail once it has been created. Therefore, any attachments received in a P7M file should be checked prior to opening, even if they are from a known and trusted source.

P7M files can be opened in any E-Mail client that supports S/MIME provided that the necessary public key is available. P7M files can also be opened using other applications such as Cryptigo p7mViewer, which is designed specifically for opening signed and encrypted E-Mails.