Which of the following is NOT a Mail Transport Agent?

Exim is a widely used open-source Mail Transport Agent (MTA) for Unix-like systems, responsible for receiving, routing, and delivering email.

Postfix is a popular open-source Mail Transport Agent (MTA), known for its security, speed, and ease of configuration, handling email delivery between servers.

Sendmail is one of the oldest and most well-known Mail Transport Agents (MTAs), responsible for transferring email messages between computers.

Qmail is a secure and reliable Mail Transport Agent (MTA), designed to be a modern replacement for Sendmail, focusing on security and modularity.

Mail Transport Agents (MTAs) like Exim, Postfix, Sendmail, and Qmail are server applications responsible for routing and delivering email between mail servers. The `mail` command (often an alias for `mailx` or part of a Mail User Agent) is a client-side utility that users employ to compose, send, and read emails, and it typically interacts with an MTA for actual delivery.