smime.p7s ... you might see on incoming emails from different senders. Sometimes when our company Super Technologies, Inc. is helping our wholesale clients to monetize IP communications, we see this. In fact, every time we receive an email from a government organization who is leveraging DID phone number service on wholesale level.

What is it?

S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for public key encryption and signing of MIME data. See more at http://en.wikipedia.org/wiki/S/MIME.