What is an MTA (Mail Transfer Agent)?

MTA (Mail Transfer Agent) is the software responsible for sending and relaying email between mail servers using SMTP. When you click Send in your mail client, the message is handed off to an MTA, which then transmits it to the recipient's MTA, which delivers it to the recipient's mailbox.

Common MTA software:

• Postfix. Most widely used on Linux servers. Descendant of Sendmail.
• Sendmail. The original. Rarely deployed for new setups but still common in legacy systems.
• Exim. Popular in shared hosting environments.
• Microsoft Exchange. Corporate MTA. Tightly integrated with Outlook and Office 365.
• MailerQ, PowerMTA. Commercial high-volume MTAs used by ESPs and large senders.

For modern senders, running your own MTA is uncommon. Reputable ESPs operate MTAs on your behalf, and that is part of what you pay for. Running your own MTA means handling sender reputation, IP warm-up, FBL subscriptions, throttling, and content scanning all yourself. The economics rarely make sense outside of very specific high-volume use cases.