SMTP Relay

Use PostStack as an SMTP relay for applications that cannot use the REST API. All features including tracking, webhooks, and analytics work with SMTP.

TLS required. The relay accepts both implicit TLS (SSL on connect) and STARTTLS on port 587 (RFC 8314). For implicit TLS set secure: true in nodemailer or use Python's smtplib.SMTP_SSL. Only unencrypted (plaintext) connections are rejected.

Connection Details

Setting	Value
Host	`smtp.poststack.dev`
Port	`587` (implicit TLS or STARTTLS)
Encryption	`TLS 1.2 / 1.3` required from connection start
Username	`poststack`
Password	Your API key (`sk_live_...`)

Mailbox users can also authenticate with their email address and mailbox password instead of an API key. See the Mailboxes docs for email client setup.

Custom Headers

Add PostStack-specific headers to your SMTP messages to use advanced features:

Header	Description
`X-PostStack-Idempotency-Key`	Prevent duplicate sends with a unique idempotency key
`X-PostStack-Tags`	Comma-separated tags for categorizing emails
`X-PostStack-Template-Id`	Use a published template by ID
`X-PostStack-Variables`	JSON-encoded template variables

Node.js (Nodemailer)

Use Nodemailer with PostStack SMTP credentials. Note secure: true — TLS is required from connection start.

typescript

import nodemailer from 'nodemailer';

const transporter = nodemailer.createTransport({
  host: 'smtp.poststack.dev',
  port: 587,
  secure: true, // implicit TLS (or use STARTTLS: secure:false, requireTLS:true)
  auth: {
    user: 'poststack',
    pass: 'sk_live_...',
  },
});

await transporter.sendMail({
  from: 'you@yourdomain.com',
  to: 'user@example.com',
  subject: 'Hello from PostStack SMTP',
  html: '<p>Sent via SMTP relay!</p>',
  headers: {
    'X-PostStack-Tags': 'welcome,onboarding',
  },
});

Python (smtplib)

Use smtplib.SMTP_SSL for implicit TLS:

bash

import smtplib
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart

msg = MIMEMultipart("alternative")
msg["From"] = "you@yourdomain.com"
msg["To"] = "user@example.com"
msg["Subject"] = "Hello from PostStack SMTP"
msg["X-PostStack-Tags"] = "welcome,onboarding"

html = "<p>Sent via SMTP relay!</p>"
msg.attach(MIMEText(html, "html"))

with smtplib.SMTP_SSL("smtp.poststack.dev", 587) as server:
    server.login("poststack", "sk_live_...")
    server.sendmail(
        "you@yourdomain.com",
        "user@example.com",
        msg.as_string(),
    )

Using Templates via SMTP

Reference templates and pass variables through custom SMTP headers:

typescript

await transporter.sendMail({
  from: 'you@yourdomain.com',
  to: 'user@example.com',
  subject: '', // Overridden by template
  text: '',    // Overridden by template
  headers: {
    'X-PostStack-Template-Id': 'tpl_abc123def456ghi789',
    'X-PostStack-Variables': JSON.stringify({
      first_name: 'Alice',
      company_name: 'Acme Inc',
    }),
  },
});