"Abstract This document specifies conventions for X.509 certificate usage by Secure/Multipurpose Internet Mail Extensions (S/MIME) v4.0 agents. S/MIME provides a method to send and receive secure MIME messages@ and certificates are an integral part of S/MIME agent processing. S/MIME agents validate certificates as described in RFC 5280 (""Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile""). S/MIME agents must meet the certificate-processing requirements in this document as well as those in RFC 5280. This document obsoletes RFC 5750."