Secure messages may be sent between senders and recipients using symmetric message keys. The symmetric message keys may be derived from a master key using a key generator at an organization. A gateway may encrypt outgoing message using the derived keys. Senders in the organization can send messages to recipients who are customers of the organization. The recipients can authenticate to a decryption server in the organization using preestablished credentials. The recipients can be provided with copies of the derived keys for decrypting the encrypted messages. A hierarchical architecture may be used in which a super master key generator at the organization derives master keys for delegated key generators in different units of the organization. An organization may have a policy server that generates non-customer symmetric message keys. The non-customer symmetric message keys may be used to encrypt messages sent by a non-customer sender to a recipient at the organization.