What is an MTA (Mail Transfer Agent)?

A Mail Transfer Agent (MTA) is software that transfers emails between mail servers using SMTP. When you hit send, your message passes through at least one MTA on the sending side and one on the receiving side before it reaches the recipient's inbox.

Open-source MTAs like Postfix, Exim, and KumoMTA handle the bulk of the world's email routing. Commercial options like PowerMTA and Halon add enterprise features such as advanced analytics and traffic shaping. Most senders never interact with an MTA directly because their ESP manages that layer behind the scenes.