What Is an SMTP Relay or Smarthost and When Do You Need One?

An SMTP relay is a mail server that accepts messages from an authorised sender and delivers them onward to their final destination. A smarthost is the same idea seen from the sending side: it is the single upstream server a device or application hands every outgoing message to, instead of contacting each recipient server itself. Every time your mail app sends through port 587, it is using its provider as a relay.

Quick summary

• Mail server (MTA): receives mail for your domain via MX and stores it in mailboxes. OquMail is this.
• SMTP relay or smarthost: the outbound half of that server, accepting your authenticated messages and delivering them. OquMail on port 587 is this too.
• Send API: the same outbound service exposed over HTTPS instead of SMTP, which is easier for code and avoids TLS negotiation and port issues.
• Open relay: a misconfigured server that forwards mail from anyone without authentication. These are blocklisted within hours; a relay you use must require a login.
• Office equipment: a multifunction printer or scanner that emails PDFs needs a smarthost because it cannot deliver directly.

How relaying works

Direct delivery means looking up the MX records (the settings that tell the internet where to deliver email for your domain) of every recipient and connecting to it on port 25. That requires a public IP with reverse DNS, an IP reputation, and a network that allows outbound 25, none of which a laptop, an office router, a printer or a small cloud server reliably has. A relay solves this: you authenticate to one trusted host, it accepts responsibility for the message, queues it, retries on temporary failures, signs it with DKIM, and delivers from IPs that receivers already trust.

Relay vs mail server vs API

• Mail server (MTA): receives mail for your domain via MX and stores it in mailboxes. OquMail is this.
• SMTP relay or smarthost: the outbound half of that server, accepting your authenticated messages and delivering them. OquMail on port 587 is this too.
• Send API: the same outbound service exposed over HTTPS instead of SMTP, which is easier for code and avoids TLS negotiation and port issues.
• Open relay: a misconfigured server that forwards mail from anyone without authentication. These are blocklisted within hours; a relay you use must require a login.

Example: pointing a device or app at a smarthost

Smarthost:   mail.oqumail.com
Port:        587
Encryption:  STARTTLS
Username:    scanner@yourdomain.com
Password:    that mailbox's password
From:        scanner@yourdomain.com  (must be on your verified domain)

When a small business needs a relay

1. Office equipment: a multifunction printer or scanner that emails PDFs needs a smarthost because it cannot deliver directly.
2. Website and app mail: contact forms, receipts and password resets. Use the OquMail API here rather than SMTP where possible.
3. A self-hosted server on a cloud provider that blocks port 25, such as most VPS plans by default.
4. Legacy software that only speaks SMTP and must send from your domain.

Common relay mistakes

• Sending From an address on a domain the relay has not verified. Receivers see a DMARC failure and the relay may refuse it.
• Using port 25 to the relay. Submission is 587; 25 is often blocked before it leaves your network.
• Sharing one mailbox password across ten devices. Create a dedicated mailbox such as devices@ so a password change does not break everything.
• Assuming the relay fixes reputation problems caused by bad lists. A relay carries your mail; it does not launder spam.

OquMail as your relay

Any OquMail mailbox can act as a smarthost for devices and legacy software using mail.oqumail.com:587 with STARTTLS. For applications, the send API at https://api.oqumail.com/api/v1/emails with a Bearer API key is the cleaner path. In both cases mail is DKIM-signed for your domain, covered by the SPF record you already published, and logged per message so you can see the remote server response when something bounces.

Common questions

Is an SMTP relay the same as an SMTP server?

A relay is a role an SMTP server plays. The same OquMail server receives inbound mail, stores it, and relays your outbound mail.

Do I need a separate relay service?

Not for normal business volumes. Your mail provider already relays for you. Dedicated relay services matter at very high volume or for special routing needs.

Why do receivers trust a relay more than my own server?

Reputation. The relay sends from stable IPs with correct reverse DNS and a long history, while a new IP has none.