Files with the .P7S extension contain e-mail messages that have been digitally signed according to the PKCS#7 standard, which stands for the Cryptographic Message Syntax Standard. A digital signature means that the message has been verified in terms of its sender and in terms of whether its content was changed during the transmission, simultaneously ensuring that it can only be read by the right recipient.

A P7S file is most often found when the e-mail software does not support digital signatures – in such cases, the message contains a smime.p7s attachment. Most software, such as Outlook or Thunderbird, does, however, allow the use of digital signatures.