If S/MIME is set to enabled, users can download the S/MIME control and use it to read and compose signed and encrypted messages. ...

If S/MIME is set to enabled, users can download the S/MIME control and use it to read and compose signed and encrypted messages. If S/MIME is set to false, users won't be able to read or compose messages that are opaque-signed or encrypted. Messages that are clear-signed can be read but not composed, and their signatures will not be validated.