What is a Mail Transfer Agent (MTA)?

A Mail Transfer Agent is software that routes and delivers email. MTAs receive messages via SMTP, queue them for delivery, attempt transmission to recipient servers, handle retries for temporary failures, and process bounces for permanent failures.

Common MTAs include: Postfix (open-source, widely deployed), Exim (flexible, common on shared hosting), qmail (security-focused legacy), PowerMTA (enterprise commercial), KumoMTA (open-source, built for high-volume delivery), and Halon (modern commercial). Your ESP likely runs multiple MTAs in their infrastructure.

For most senders, MTAs are invisible, running inside ESP and hosting provider systems. Advanced senders managing their own infrastructure configure MTAs directly, tuning performance, connection limits, and delivery optimization for their specific needs.