SMTP relay

Legacy app, framework mailer, or a tool you can’t change? The SMTP relay accepts standard SMTP and funnels it through the exact same pipeline as the REST API: quota, suppression checks, and tracking all apply.

Configuration

Setting	Value
Host	smtp.sendsignal.dev
Port	587
Encryption	STARTTLS
Authentication	AUTH PLAIN
Username	anything (ignored)
Password	your API key (ss_...)
Max message size	10 MiB

The username is ignored; the API key in the password field is the credential. Use a sending_access key here rather than a full-access one.

Example: Nodemailer

import nodemailer from "nodemailer";

const transport = nodemailer.createTransport({
  host: "smtp.sendsignal.dev",
  port: 587,
  secure: false, // STARTTLS upgrade
  auth: {
    user: "sendsignal",
    pass: process.env.SENDSIGNAL_API_KEY,
  },
});

await transport.sendMail({
  from: "Acme <hi@acme.com>",
  to: "user@example.com",
  subject: "Hello",
  html: "<p>Hi</p>",
});

What happens to relayed mail

The relay parses your MIME message (subject, HTML and text bodies) and creates a normal SendSignal email: it shows up in the console, emits the same delivery events, honors your suppression list and monthly quota, and gets tracking and unsubscribe handling per your domain settings. From the platform’s point of view there is no difference between an SMTP send and a POST /emails.