{"id":79562,"date":"2024-04-23T19:56:14","date_gmt":"2024-04-23T19:56:14","guid":{"rendered":"https:\/\/mailrelay.com\/glosario\/mta-mail-transfer-agent\/"},"modified":"2026-10-07T15:34:21","modified_gmt":"2026-10-07T15:34:21","slug":"mta-mail-transfer-agent","status":"publish","type":"glossary","link":"https:\/\/mailrelay.com\/en\/glossary\/mta-mail-transfer-agent\/","title":{"rendered":"MTA (Mail Transfer Agent)"},"content":{"rendered":"\n

An MTA (Mail Transfer Agent) is the program that receives an email and sends it on to the next server until it reaches the recipient’s server.<\/strong> Put simply, it’s the agent that transfers your mail.<\/p>\n\n

It’s the internet’s mail carrier: you write the message, but it’s the one that delivers it.<\/p>\n\n

Every email you send passes through at least two of these agents: your provider’s and the recipient’s. They talk to each other using the SMTP<\/a> protocol.<\/p>\n\n

What does MTA mean?<\/h2>\n\n

In email, MTA stands for Mail Transfer Agent<\/em>. You’ll also see Mail Transport Agent<\/em>, which means the same thing.<\/p>\n\n

But the acronym has other meanings, depending on the context:<\/p>\n\n

Context<\/th>What MTA means<\/th><\/tr><\/thead>
Email<\/td>Mail Transfer Agent: the program that carries emails between servers<\/td><\/tr>
Chats and WhatsApp<\/td>A vulgar slang abbreviation, common in Mexico, used as an exclamation<\/td><\/tr>
Transportation<\/td>Metropolitan Transportation Authority, New York’s public transit authority<\/td><\/tr>
Certifications<\/td>Microsoft Technology Associate, a Microsoft certification retired in 2022<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

In this glossary, we’re talking about the first one.<\/p>\n\n

How does an MTA work?<\/h2>\n\n

When you hit “Send”, your message goes through these steps:<\/p>\n\n

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  1. Your email client hands the message to your provider’s server, usually on port 587 and with a username and password.<\/li>\n\n\n\n
  2. Your provider’s server queries DNS for the MX record of the recipient’s domain, which says which server receives their mail.<\/li>\n\n\n\n
  3. It connects to that server over SMTP, on port 25<\/a>, and transfers the message, encrypted with TLS<\/a> if both support it.<\/li>\n\n\n\n
  4. If the destination server doesn’t respond, the sending server keeps the message in a queue and keeps retrying for hours or days. If it’s rejected permanently, it returns a bounce<\/a>.<\/li>\n\n\n\n
  5. The destination server accepts the message and drops it in the recipient’s mailbox, where they read it via IMAP<\/a> or POP3<\/a>.<\/li>\n<\/ol>\n\n
    \"An<\/figure>\n\n

    Sometimes the message passes through more intermediate servers.<\/p>\n\n

    When a server receives an email and forwards it to another, it’s said to act as a relay<\/em>. We explain this in our entry on relay mail<\/a>.<\/p>\n\n

    MTA, MUA, MDA and MSA: the differences<\/h2>\n\n

    The transfer agent is just one piece of the system. These are the others:<\/p>\n\n

    Agent<\/th>Full name<\/th>What it does<\/th>Example<\/th><\/tr><\/thead>
    MUA<\/td>Mail User Agent<\/td>The program you use to write and read email<\/td>Outlook, Thunderbird or the Gmail app<\/td><\/tr>
    MSA<\/td>Mail Submission Agent<\/td>Receives the message from your email client and checks that you’re allowed to send<\/td>Your provider’s server on port 587<\/td><\/tr>
    MTA<\/td>Mail Transfer Agent<\/td>Carries the message from server to server<\/td>Exim, Postfix or Sendmail<\/td><\/tr>
    MDA<\/td>Mail Delivery Agent<\/td>Drops the message in the recipient’s mailbox<\/td>Dovecot or Procmail<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

    In practice, a single email server<\/a> usually handles the last three roles at once.<\/p>\n\n

    That’s why people often say MTA when they mean the whole server.<\/p>\n\n

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    MTA examples<\/h2>\n\n

    These are some of the best-known transfer agents:<\/p>\n\n

    MTA<\/th>Type<\/th>Note<\/th><\/tr><\/thead>
    Exim<\/a><\/td>Free software<\/td>The most widespread: a March 2023 E-Soft survey put it at around 59% of public mail servers<\/td><\/tr>
    Postfix<\/td>Free software<\/td>Widely used on Linux servers for its security and simple configuration<\/td><\/tr>
    Sendmail<\/a><\/td>Free software<\/td>One of the first, created by Eric Allman in the 1980s<\/td><\/tr>
    Microsoft Exchange Server<\/td>Commercial<\/td>Microsoft’s mail server for businesses<\/td><\/tr>
    PowerMTA<\/td>Commercial<\/td>Built for sending large volumes, widely used by email marketing platforms<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

    For office email, a well-configured free software program is enough.<\/p>\n\n

    But bulk sending needs one that’s built to handle millions of messages, per-provider queues and thousands of bounces.<\/p>\n\n

    Why does the MTA matter in email marketing?<\/h2>\n\n

    If you send newsletters, the MTA determines much of your deliverability<\/a>. These are its most important tasks:<\/p>\n\n