smime-CA-directory

smime-CA-directory is a variable defined in `smime.el'.
Its value is

nil

Documentation:
*Directory containing certificates for CAs you trust.
Directory should contain files (in PEM format) named to the X.509
hash of the certificate. This can be done using OpenSSL such as:

$ ln -s ca.pem `openssl x509 -noout -hash -in ca.pem`.0

where `ca.pem' is the file containing a PEM encoded X.509 CA
certificate.

You can customize this variable.