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), 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.
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