With DomainKeys or DKIM email authentication, the sender signs the email using public key cryptography. Configuring DomainKeys and DKIM Signing A signing key is the private key stored on the appliance. https://www.cisco.com/c/en/us/td/docs/security/esa/esa11-1/user_guide/b_ESA_Admin_Guide_11_1/b_ESA_Admin_Guide_chapter_010101.html?bookSearch=true