A security team informs the Linux administrator that a specific server is acting as an open relay. To which of the following is the security team referring?

An SSH service exposed to the internet is a remote access security concern, not an open relay - the term 'relay' applies exclusively to email (SMTP) message forwarding, not remote shell sessions.

An unmanaged router forwarding traffic describes a network layer routing misconfiguration, which is categorically different from an open relay, a term reserved for application-layer email routing.

An unauthenticated web proxy that forwards HTTP/HTTPS traffic is called an open proxy, not an open relay - these are distinct misconfigurations at different protocol layers.

An open relay is specifically an SMTP service that accepts messages from unauthorized external senders and relays them to recipients on other domains without requiring authentication. This misconfiguration allows spammers to route bulk email through the server, obscuring the true origin and consuming resources. Properly hardened mail servers restrict relaying to authenticated users or explicitly whitelisted IP addresses.