There is only one file type that uses the .P7M extension:
Digitally Encrypted Message
A P7M file is an encrypted email message that contains the contents and attachments of a sensitive email. It is used to prevent unauthorized users from accessing information in an email and may require a private key that matches the message's public key in order to open the document. P7M files are typically used for protecting confidential medical, financial, and business information.
P7M files are typically seen as smime.p7m files. These files include the contents and attachments of an email and have been secured with Secure/Multipurpose Internet Mail Extensions (S/MIME), which is a public key encryption standard of MIME data.
Most P7M files, including smime.p7m files, can be opened using Cryptigo p7mViewer.
Updated: April 19, 2017