{"id":81836,"date":"2024-09-12T18:01:08","date_gmt":"2024-09-12T18:01:08","guid":{"rendered":"https:\/\/mailrelay.com\/glossary\/relay-mail\/"},"modified":"2026-10-06T23:39:39","modified_gmt":"2026-10-06T23:39:39","slug":"relay-mail","status":"publish","type":"glossary","link":"https:\/\/mailrelay.com\/en\/glossary\/relay-mail\/","title":{"rendered":"Relay Mail"},"content":{"rendered":"\n

A mail relay, or SMTP relay, is a server that receives emails from your website or app and delivers them to their recipients.<\/strong><\/p>\n\n

You hand it the message and it does the rest: it authenticates it, queues it, delivers it and handles bounces.<\/p>\n\n

To relay<\/em> means to receive something and pass it on. That’s why you’ll also see it called email relaying, a relay server or simply a mail relay<\/em>.<\/p>\n\n

What is a mail relay?<\/h2>\n\n

In a broad sense, any server that receives an email and sends it on to another server is acting as a relay.<\/p>\n\n

That’s what every MTA<\/a> does along an email’s path to the inbox.<\/p>\n\n

But today, when people talk about an SMTP relay, they almost always mean a service that specializes in sending email on behalf of others<\/strong>.<\/p>\n\n

For example, the server an online store uses to send its order confirmations, or the one a contact form uses to notify you of a new message.<\/p>\n\n

The name comes from the SMTP<\/a> protocol, which all mail servers use to communicate with each other.<\/p>\n\n

How does an SMTP relay work?<\/h2>\n\n

This is the journey of an email sent through a relay service:<\/p>\n\n

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  1. Your app connects to the relay server, usually on port 587 or 465, and identifies itself with a username and password, an API key or its IP address.<\/li>\n\n\n\n
  2. The relay checks that you’re allowed to send with that sender<\/a> and signs the message with DKIM<\/a>.<\/li>\n\n\n\n
  3. It puts the message in a queue and looks up the MX record of the recipient’s domain.<\/li>\n\n\n\n
  4. It delivers it to the destination server on port 25, encrypted with TLS<\/a> if the other server supports it.<\/li>\n\n\n\n
  5. If it can’t deliver it, it retries or records the bounce<\/a>. And it shows you delivery, open and click statistics.<\/li>\n<\/ol>\n\n
    \"How<\/figure>\n\n

    The advantage is that your app doesn’t need to know anything about queues, retries or reputation. All it has to do is hand the message over to the relay.<\/p>\n\n

    Relay server, relay host and open relay: the differences<\/h2>\n\n

    There are several terms around mail relays that are often confused. Here’s how they differ:<\/p>\n\n

    Term<\/th>What it is<\/th>Example<\/th><\/tr><\/thead>
    Relay server<\/td>Any server that receives email and forwards it to another server<\/td>Your email provider’s outgoing server<\/td><\/tr>
    Relay host or smarthost<\/td>The server your own server sends all outgoing email to, instead of delivering it directly<\/td>A Postfix server configured to send through an external service<\/td><\/tr>
    SMTP relay (service)<\/td>A provider that makes its sending infrastructure available to you<\/td>Mailrelay, SendGrid or Amazon SES<\/td><\/tr>
    Open relay<\/td>A server that accepts and forwards email from anyone, without authentication<\/td>A misconfigured server that ends up sending spam<\/td><\/tr>
    Relay domain<\/td>A domain for which a server accepts email and passes it on to another server<\/td>A company’s domain whose email goes through a filter before reaching its mailboxes<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

    The relay host deserves a closer look.<\/p>\n\n

    If your company has its own mail server, you can point it to a relay host so it doesn’t deliver the messages itself.<\/p>\n\n

    That way it takes advantage of a specialized provider’s reputation and infrastructure. In Postfix, for example, it’s set with the relayhost<\/em> parameter.<\/p>\n\n

    You’ll find more details on the last term in our entry on relay domains<\/a>.<\/p>\n\n

    What is an open relay and why is it dangerous?<\/h2>\n\n

    An open relay is a server that forwards email from anyone, without asking for a username or password. In the early years of the internet, this was the norm.<\/p>\n\n

    But spammers started using them to send millions of messages without leaving a trace.<\/p>\n\n

    Today an open relay lands on blacklists<\/a> within a few hours, and all email from that server gets blocked.<\/p>\n\n

    That’s why any serious relay requires authentication. We explain it in detail in our entry on open mail relays<\/a>.<\/p>\n\n

    When do you need an SMTP relay service?<\/h2>\n\n

    You need a relay whenever one of your systems sends emails automatically. These are the most common cases:<\/p>\n\n

    Situation<\/th>Problem without a relay<\/th>What the relay provides<\/th><\/tr><\/thead>
    Forms on a WordPress<\/a> website<\/td>The hosting’s mail function doesn’t authenticate properly and notifications end up in spam<\/td>Authenticated sending with SPF and DKIM<\/td><\/tr>
    WooCommerce or PrestaShop<\/a> online store<\/td>Order and shipping confirmations that never arrive<\/td>Fast delivery and statistics for every transactional email<\/a><\/td><\/tr>
    CRM<\/a>, ERP<\/a> or your own app<\/td>You have to set up and maintain a mail server<\/td>Connection via SMTP or API in minutes<\/td><\/tr>
    Cloud servers<\/td>Google Cloud blocks outbound port 25 and AWS restricts it by default<\/td>Sending through port 587 or 465<\/td><\/tr>
    Printers and scanners that send by email<\/td>Older devices don’t support modern authentication methods<\/td>A relay that accepts their connection securely<\/td><\/tr>
    High volumes<\/td>Gmail limits personal accounts to 500 emails a day<\/td>Thousands or millions of emails a day<\/td><\/tr><\/tbody><\/table><\/figure>\n\n
    \n

    Are the emails from your website or store not arriving?<\/strong><\/p>\n\n\n\n

    Connect your website, store or app to Mailrelay’s SMTP and check the statistics for every email you send.<\/p>\n\n\n\n

    \n
    Create a free account<\/a><\/div>\n<\/div>\n<\/div><\/div>\n\n

    How to set up an SMTP relay<\/h2>\n\n

    Almost all programs and plugins ask for the same details to connect to a relay:<\/p>\n\n

    Setting<\/th>What to enter<\/th><\/tr><\/thead>
    SMTP server<\/td>The address your provider gives you, for example smtp.provider.com<\/em><\/td><\/tr>
    Port<\/td>587 with STARTTLS, which is recommended, or 465 with SSL\/TLS. Some providers also offer 2525<\/td><\/tr>
    Authentication<\/td>Username and password, or an API key<\/td><\/tr>
    Sender<\/td>An address on your own domain, never Gmail or Yahoo<\/td><\/tr>
    DNS records<\/td>The SPF<\/a> and DKIM<\/a> records your provider gives you, plus a DMARC<\/a> policy<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

    If you’re not sure which port to use, our entry on the SMTP port<\/a> explains the differences between 25, 465, 587 and 2525.<\/p>\n\n

    \"Settings<\/figure>\n\n

    SMTP relay options in 2026<\/h2>\n\n

    These are some of the most widely used services, with data as of September 2026:<\/p>\n\n

    Service<\/th>Best for<\/th>Free offering<\/th>Paid pricing<\/th><\/tr><\/thead>
    Mailrelay<\/td>Websites, stores and apps that also send campaigns<\/td>30-day trial on the free account<\/td>Included in the Standard and Enterprise plans, or \u20ac14.95 a month on the free account<\/td><\/tr>
    Google Workspace<\/td>Companies already using Google Workspace<\/td>Included in the subscription<\/td>Up to 10,000 recipients a day per user<\/td><\/tr>
    Amazon SES<\/a><\/td>Technical teams with high volumes<\/td>No free tier for new accounts since July 2025<\/td>$0.10 per 1,000 emails<\/td><\/tr>
    SendGrid<\/a><\/td>High-traffic apps<\/td>60-day trial with 100 emails a day<\/td>From $19.95 a month<\/td><\/tr>
    Mailgun<\/a><\/td>Developers<\/td>100 emails a day<\/td>From $15 a month for 10,000 emails<\/td><\/tr>
    Brevo<\/a><\/td>Small volumes<\/td>300 emails a day<\/td>From $9 a month<\/td><\/tr>
    Postmark<\/td>Very fast transactional emails<\/td>100 emails a month<\/td>From $15 a month for 10,000 emails<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

    Before choosing, check three things: whether the service keeps your transactional emails separate from your marketing ones, whether it gives you statistics and whether it offers support in your language.<\/p>\n\n

    You’ll find more criteria in our entry on SMTP services<\/a>.<\/p>\n\n

    SMTP relay or your own mail server?<\/h2>\n\n

    You can set up your own sending server with Postfix or Exim. But for most companies it isn’t worth it:<\/p>\n\n

    Aspect<\/th>Your own server<\/th>SMTP relay service<\/th><\/tr><\/thead>
    Setup<\/td>Install, configure and secure the server<\/td>Create an account and copy the connection details<\/td><\/tr>
    Reputation<\/td>You start from scratch with a new IP<\/td>You use IPs with a good reputation or an already warmed-up dedicated IP<\/td><\/tr>
    Maintenance<\/td>Updates, queues, blacklists and bounces are all on you<\/td>The provider takes care of it<\/td><\/tr>
    Statistics<\/td>You have to build them yourself<\/td>Deliveries, opens, clicks and bounces from day one<\/td><\/tr>
    Cost<\/td>A server plus a technician’s time<\/td>A monthly fee based on volume<\/td><\/tr><\/tbody><\/table><\/figure>\n\n

    Your own server only makes sense if you have a dedicated technical team and very specific privacy or volume requirements. In all other cases, a relay saves you time and delivery problems.<\/p>\n\n

    SMTP relay and deliverability<\/h2>\n\n

    A relay alone doesn’t guarantee that your emails reach the inbox.<\/p>\n\n

    But it makes it much easier to meet what mailbox providers require. These guidelines will help you improve your deliverability<\/a>:<\/p>\n\n