relayservers

Relay servers are servers that receive messages or traffic and forward them toward their destination rather than generating the content themselves. They act as intermediaries in communications, helping to route, filter, or transform data as it passes through a network. The term is used in several contexts, most commonly for email relays and for network proxies.

Email relay servers (SMTP relays) accept outgoing mail from clients or other mail servers for a given domain, apply policy and filtering, and forward the messages to the recipient's mail server. They can be used by Internet service providers, hosting companies, or organizations to centralize mail handling, enforce anti-spam controls, and provide authentication and TLS encryption in transit. In mail systems, relays are typically restricted by authentication requirements or domain-based rules to prevent abuse. Open relays that allow unauthenticated relay are considered a serious security risk and are generally disabled.

Network relay servers act as forward or reverse proxies for client requests. A forward proxy relays client requests to target servers, often enforcing access controls or caching. A reverse proxy sits in front of one or more backend services, handling TLS termination, load balancing, and request shaping. Relay capabilities also exist in VoIP servers, content delivery networks, and other messaging systems where traffic is routed through intermediate nodes.

Security and reliability are central concerns. Operators implement authentication (for email relays), encryption (TLS), access controls, rate limiting, monitoring, and redundancy. Misconfiguration, especially open relays or weak authentication, can enable abuse or outages. Typical software for email relays includes Postfix, Exim, and Microsoft Exchange; proxy relays are implemented by servers such as Nginx, HAProxy, or Squid.

See also: SMTP relay, proxy server, open relay, TLS, anti-spam measures.