S/MIME is a private/public key pair system which provides digital signatures, message privacy, and tamper detection in a open, interoperable way. S/MIME can provide message privacy by encrypting messages that only the sender and the intended recipients can decode. It allows you to identify the sender of a message via a digital signature. Together, this encryption and digital signing help provide tamper detection, allowing recipients to determine whether a message was altered after it was sent.
If this option is selected, you will be able to import your personal S/MIME keys, read encrypted mail, verify digitally signed mail, and encrypt/sign your outgoing mail.