SMTP vs API for Sending Email

SMTP vs API for Sending Email

Feature	SMTP Relay	Email API (REST)
Protocol	SMTP (port 587 or 465)	HTTPS (REST with JSON)
Authentication	Username and password	API key in Authorization header
Integration	Change SMTP config — no code	Add HTTP calls to your app
Speed	Connection overhead per session	Single HTTP request per email
Features	Basic send only	Templates, scheduling, priority
Best for	Legacy apps, CMS plugins, Rails	Modern apps, high-volume sends
Deliverability	Same infrastructure	Same infrastructure

Both SMTP and API deliver through the same email infrastructure with the same deliverability. The choice depends on your application architecture and integration preferences.

When to Use SMTP

SMTP relay is the right choice when:

• Your application already has SMTP configuration (Rails, Django, Laravel, WordPress)
• You want to switch email providers without changing application code
• You need to integrate with legacy systems that only support SMTP
• Your CMS or framework has built-in SMTP support

With SMTP, you change configuration settings — no code changes required. RelayPost provides SMTP relay on port 587 (STARTTLS) and 465 (TLS).

When to Use an Email API

A REST API is the right choice when:

• You are building a new application and want full control over email sending
• You need advanced features like templates, scheduling, or priority levels
• You send high volumes and want the efficiency of single HTTP requests
• You want structured JSON responses with delivery status and message IDs

The API gives you programmatic control over every aspect of email sending, including template rendering, metadata, and scheduling.

Code Comparison

Sending via SMTP (Nodemailer)

const nodemailer = require("nodemailer");

const transporter = nodemailer.createTransport({
  host: "smtp.relaypost.dev",
  port: 587,
  secure: false,
  auth: { user: "smtp_username", pass: "smtp_password" },
});

await transporter.sendMail({
  from: "[email protected]",
  to: "[email protected]",
  subject: "Hello",
  html: "<p>Sent via SMTP relay</p>",
});

Sending via API (fetch)

const response = await fetch("https://api.relaypost.dev/v1/emails/send", {
  method: "POST",
  headers: {
    "Content-Type": "application/json",
    Authorization: "Bearer YOUR_API_KEY",
  },
  body: JSON.stringify({
    from: { email: "[email protected]", name: "Your App" },
    to: [{ email: "[email protected]" }],
    subject: "Hello",
    html: "<p>Sent via REST API</p>",
  }),
});

Frequently Asked Questions

Is SMTP or API faster for sending email?

The REST API is generally faster for individual sends because each email is a single HTTP request. SMTP requires establishing a connection, performing a TLS handshake, and exchanging multiple commands per session. For batch sending, SMTP can reuse a single connection for multiple messages, which reduces overhead.

Can I use both SMTP and API with the same provider?

Yes. RelayPost supports both SMTP relay and REST API on the same account. Both methods deliver through the same infrastructure with the same deliverability. You can use SMTP for your legacy applications and the API for new integrations.

Does SMTP or API have better deliverability?

Deliverability is identical. Both SMTP and API route through the same sending infrastructure, IP addresses, and DKIM signing. The delivery method does not affect inbox placement — authentication (SPF, DKIM, DMARC) and sender reputation are what matter.