Account binding is the backend process that links a privacy alias, relay address, or pseudonymous identifier to the real user account or mailbox that should receive messages. In email privacy systems, the alias looks separate from the destination inbox, but the service must still know where to forward mail. That hidden mapping is the control point that makes the feature work.
In cyber security, account binding matters because it is a privacy boundary. If the binding is exposed, guessed, or modified, an attacker can deanonymize the user, correlate activity across services, or send targeted phishing to the real mailbox. Defenders protect this layer with strict access controls, minimized data exposure, authenticated lookup paths, and careful logging and segregation of alias records. A flaw in binding logic may not break delivery, but it can silently defeat the privacy promise behind the system.