SMTP Relay
Send through useSend from any app that speaks SMTP, using the optional smtp-proxy container that forwards to the instance API.
useSend ships an optional SMTP proxy — the Docker image usesend/smtp-proxy — that accepts standard SMTP connections and forwards each message to the useSend instance API. Use it for frameworks and apps that only speak SMTP: Nodemailer, Supabase Auth, Django, Rails, WordPress, and similar.
The proxy is a thin translator. It does not touch SES directly — it authenticates the SMTP session, then calls the same POST /api/v1/emails endpoint the Send Email API uses. Everything downstream (domain verification, DKIM/SPF/DMARC, SES sending) is identical to the API path.
For most EverJust products, prefer the HTTP API over SMTP. The API is simpler to secure, easier to observe, and avoids the port-25 pitfalls below. Reach for SMTP only when a third-party app cannot be configured to call an HTTP API.
How it works
Your app opens an SMTP connection to the proxy and authenticates with username usesend and a useSend API key (the us_ value) as the password.
The proxy validates the credentials and reads the message (envelope, headers, body, attachments).
The proxy calls the useSend instance API at USESEND_BASE_URL to enqueue the send. The email is delivered through Amazon SES exactly as an API-submitted message would be.
Running the proxy
The proxy is separate infrastructure from the main useSend deployment. Run it as its own container and point it at the instance base URL.
docker run -d \
--name usesend-smtp-proxy \
-e USESEND_BASE_URL=https://mail.everjust.app \
-e SMTP_AUTH_USERNAME=usesend \
-p 25:25 \
-p 587:587 \
-p 2587:2587 \
-p 465:465 \
-p 2465:2465 \
usesend/smtp-proxyUSESEND_BASE_URL— the useSend instance the proxy forwards to. For EverJust this ishttps://mail.everjust.app.SMTP_AUTH_USERNAME— the fixed SMTP username clients must present. Set tousesend.
The API key is not configured on the proxy. Each client supplies its own us_ key as the SMTP password, so a single proxy can serve any Team on the instance.
The proxy must be network-reachable from whatever app connects to it. If you deploy it behind a firewall or in a private subnet, ensure the SMTP ports are open to your app. This is a distinct service from https://mail.everjust.app and has its own uptime.
Ports
| Port | Transport | Notes |
|---|---|---|
| 25 | STARTTLS | Standard SMTP. Often blocked outbound — see below. |
| 587 | STARTTLS | Standard submission port. Preferred for STARTTLS. |
| 2587 | STARTTLS | Alternate submission port for hosts that block 587. |
| 465 | Implicit TLS | TLS negotiated at connection (SMTPS). |
| 2465 | Implicit TLS | Alternate implicit-TLS port. |
Choose implicit TLS (465 / 2465) or STARTTLS (587 / 2587) based on what your client library supports. The 2xxx variants exist so the proxy can run somewhere that already occupies the standard ports.
AWS blocks outbound TCP port 25 by default on EC2 and Lambda. If your sending app runs on AWS, connecting to the proxy on port 25 will time out. Use 587/2587 (STARTTLS) or 465/2465 (implicit TLS) instead — and for AWS-hosted senders, the HTTP API avoids the problem entirely.
Credentials
| Setting | Value |
|---|---|
| Host | your proxy host (e.g. smtp.everjust.app) |
| Port | 587 or 2587 (STARTTLS), 465 or 2465 (implicit TLS) |
| Username | usesend |
| Password | a useSend API key, e.g. us_xxx |
Treat the API key as a secret — it grants the same sending rights as the HTTP API. Use a per-product key from the product's own Team so sends are correctly attributed and revocable.
Self-hosted useSend has rate limiting disabled, so the proxy will not throttle submissions. Throughput is bounded by your SES account limits. Note also that SES is currently in sandbox: you can only send to verified addresses or the SES mailbox simulator (success@simulator.amazonses.com, bounce@, complaint@) until AWS grants production access.
Nodemailer example
import nodemailer from "nodemailer";
const transporter = nodemailer.createTransport({
host: "smtp.everjust.app",
port: 465, // implicit TLS
secure: true, // true for 465, false for 587/2587 (STARTTLS)
auth: {
user: "usesend",
pass: process.env.USESEND_API_KEY, // us_xxx
},
});
await transporter.sendMail({
from: "EverJust <hello@send.everjust.app>",
to: "success@simulator.amazonses.com", // verified address while in sandbox
subject: "Hello from useSend SMTP",
text: "Sent through the useSend SMTP proxy.",
html: "<p>Sent through the useSend SMTP proxy.</p>",
});For STARTTLS on port 587 (or 2587), set port: 587 and secure: false — Nodemailer will upgrade the connection with STARTTLS automatically.
Generic SMTP settings
Any SMTP-capable app (Django's EMAIL_* settings, Rails Action Mailer, Supabase custom SMTP, WordPress plugins, etc.) can be pointed at the proxy with these values:
Host: smtp.everjust.app
Port (STARTTLS): 587 (or 2587)
Port (TLS): 465 (or 2465)
Encryption: STARTTLS or SSL/TLS (match the port)
Username: usesend
Password: us_xxx # your useSend API key
From address: hello@send.everjust.app # must be on a verified domainThe From address must use a verified sending domain on the Team the API key belongs to — for EverJust, send.everjust.app. Sends from unverified domains are rejected.
Troubleshooting
Migrate from Resend
Move your product from Resend to self-hosted useSend with a near-identical per-email API and no per-domain fees.
MCP Server
Run the useSend MCP server so AI agents like Claude Code, Claude Desktop, and Cursor can send email and manage domains, contacts, campaigns, and analytics on the EverJust instance.