Transactional email, SMTP, Deliverability
What Is an SMTP Relay and When Do You Need One?
An SMTP relay is a mail server that takes the email your app or device hands it and delivers it to the recipient on your behalf. Here is how that works, what to set up so the mail is trusted, and when to skip SMTP and use an API instead.
7 min read
In this article
An SMTP relay is a mail server that accepts an email from your application, website or device over SMTP and delivers it to the recipient's mail server for you. Your software never has to find the recipient's server, retry a failed delivery or build a sending reputation of its own: the relay does all three. You connect to it with a hostname, a port, and a username and password (or an API key used as the password).
How an SMTP relay works
- Your app connects to the relay's SMTP server and authenticates, so the relay knows the mail is yours and not a stranger's.
- It hands over the message: the sender address, the recipients, the subject and the body.
- The relay looks up each recipient domain's MX record, connects to that mail server and delivers the message, retrying later if the server says to try again.
- If a delivery fails for good (the address does not exist, or the recipient's server rejects it), a well-run relay records the bounce and stops sending to that address.
That is the whole job. A relay is not a mailbox: it does not store mail for people to read. A mail server with mailboxes (Gmail, Microsoft 365) receives and stores email; a relay only sends it on.
Which port to use
| Port | Used for | Notes |
|---|---|---|
| 587 | Authenticated submission | The standard choice for a relay. Starts as plain text and upgrades to TLS with STARTTLS. |
| 465 | Authenticated submission over implicit TLS | Encrypted from the first byte. Fine where a client only supports it. |
| 2525 | An alternative submission port | Offered by some providers for networks that block 587. Check that yours supports it. |
| 25 | Server-to-server delivery | Not for apps. Many hosting and cloud providers block or throttle outbound port 25 by default. |
Why use a relay instead of sending from your own server
- Delivery is the hard part. Inbox providers judge mail by the sending IP and domain, and a relay's established IPs and feedback loops are something a fresh server does not have.
- Outbound port 25 is often blocked on cloud hosts, so a server of your own may not be able to reach recipients at all.
- Retries, bounce handling and suppression are built in, so you do not write that code.
- Your own server's IP is not exposed to blocklists when something goes wrong.
What to set up so the mail is trusted
A relay delivers the mail, but the receiving server decides whether to believe it. Three DNS records on the domain in your From address decide that:
- SPF lists which servers may send for your domain. Add the relay's include to your record, and keep to one SPF record with no more than ten DNS lookups.
- DKIM signs each message so the receiver can check it was not altered. The relay gives you the records to publish, usually CNAMEs.
- DMARC tells receivers what to do when SPF and DKIM fail to line up with your From domain. Start with p=none and a reporting address, then tighten it once the reports look clean.
Gmail and Yahoo require SPF or DKIM from every sender, and DMARC with alignment from anyone sending in bulk. You can check your records for free with the SPF checker, the DKIM checker and the DMARC checker.
SMTP relay or an email API?
Most email providers offer both. They end up in the same place, but they are used differently.
| SMTP relay | Email API | |
|---|---|---|
| How you send | A mail library speaks SMTP to the relay | An HTTPS request with a JSON body |
| Works with | Anything that can send SMTP: WordPress, CMS plugins, printers, legacy apps | Code you write or can change |
| Errors | Reported by SMTP reply codes, often after the fact | Returned straight away in the response |
| Extras | Whatever the provider adds on top | Usually richer: tags, templates, per-message status, webhooks |
Use an SMTP relay when the software you are connecting only knows how to send through SMTP, or when you are moving an existing app and want the smallest change. Use an API for new code, where the immediate, structured response is easier to work with.
Choosing a provider
- Check that it offers SMTP at all: some providers are API only.
- Look at what the plan includes at your monthly volume, and what overage costs, using the provider's own pricing page.
- Check how long it keeps logs and how it handles bounces and complaints.
- Ask whether a dedicated IP is available if you send a lot.
Open relays: the mistake to avoid
A relay that accepts mail from anyone, with no authentication, is called an open relay. Spammers find and abuse them within hours, and the server's IP ends up on blocklists. If you run your own relay, require authentication, restrict who may connect, and never leave it open to the internet. If you use a provider's, the authentication is already handled.
Frequently asked questions
What is an SMTP relay?
A mail server that accepts email from your application or device over SMTP and delivers it to the recipient's mail server on your behalf, handling retries and bounces. It sends mail; it does not store mailboxes.
What port should I use for SMTP relay?
Port 587 with STARTTLS is the standard for authenticated sending. Port 465 uses implicit TLS and is fine where a client requires it. Port 25 is for server-to-server delivery and is often blocked by hosting providers.
What is the difference between an SMTP relay and an SMTP server?
People often use the words interchangeably. Strictly, an SMTP server is any server that speaks SMTP, including the one that receives and stores your mail. A relay is one that accepts mail from a sender and passes it on toward the recipient.
Do I need SPF and DKIM when I use an SMTP relay?
Yes. The relay delivers the message, but the receiving server uses SPF, DKIM and DMARC on your From domain to decide whether to trust it. Publish the records your provider gives you before you send.
Should I use an SMTP relay or an API?
Use SMTP when the software you are connecting can only send through SMTP, or when you want the smallest change to an existing app. Use an API for new code, because the response is immediate and structured.