A relay domain is the internet domain used by a privacy relay or email alias service to receive messages and forward them to the real destination without exposing that destination address. The sender sees the relay domain, while the service privately maps the message to the underlying mailbox or account.
Relay domains matter because they are part of the trust boundary for anonymization. If the domain, routing rules, or mapping layer are misconfigured, an attacker may infer the real address, correlate aliases, or bypass the privacy layer through enumeration or logic flaws. Defenders use relay domains with strict backend mapping, validation, logging, and mail authentication controls such as SPF, DKIM, and DMARC. In real attacks, relay infrastructure is often probed for weak redirects, predictable patterns, or leaks in headers and error messages that can deanonymize users.