A Mail Transfer Agent (MTA) is software that routes email between systems, decides where messages should go, and delivers them to the next server or final destination. Common MTAs run on Unix-like systems and handle tasks such as message queuing, relay policy, and protocol negotiation with other mail servers.
MTAs matter in cyber security because they sit at a trust boundary: they process external input while often running with elevated permissions and access to sensitive mail data. Attackers may target an MTA to abuse parsing bugs, bypass routing controls, steal messages, or gain privilege escalation on the host. Defenders harden MTAs by patching quickly, restricting who can submit or relay mail, isolating services, reviewing configuration, and monitoring logs for unusual delivery patterns or privilege-related errors.