Specifies message signature type.
There are two possible types of message signing.
If the first type is used (property value is true) any user will be able to read the message.
If program that can't process signatures is used there is no way to check signature but it is still possible to read the message.
When this type is used content-type of the message is 'multipart/signed'.
When second type is used (property value is true), such program will be only able to show attachment with no meaning. But the second type is more protected against damages during passing the mail servers. When this type is used, content-type of the message is 'application/pksc7-mime; smime-type=signed-data'. Default value is True which corresponds to the first type.