Attempts to sign the entity MIME part with userid's private key using S/MIME. If successful, a new application/pkcs7-mime object is returned.
a #GMimeObject
the user id to sign with
a new #GMimeApplicationPkcs7Mime object on success or null on fail. If signing fails, an exception will be set on err to provide information as to why the failure occurred.
ErrorWrap
Attempts to sign the entity MIME part with userid's private key using S/MIME. If successful, a new application/pkcs7-mime object is returned.