All public keys and certificates must also be stored to an LDAP directory, accessible by the Sun Java System Directory Server. This is referred to as publishing the public keys so they are available to other mail users who are creating S/MIME messages.
Public keys of the sender and receiver are used in the encrypting-decrypting process of an encrypted message. Public key certificates are used to validate private keys that were used for digital signatures.
See 24.11 Managing Certificates for more information to use ldapmodify to publish the public keys and certificates.