Docs API Reference SMTP Relay

SMTP Relay

Already have a mail library? Point it at our SMTP relay and your transactional email starts flowing — no API client to install.

When to use SMTP

Reach for SMTP if you:

Already use a mail library (Nodemailer, SwiftMailer, ActionMailer, JavaMail, MailKit, etc.) and don’t want to swap it out
Want to migrate from Postfix, SendGrid, Mailgun, or another SMTP provider with zero code changes — just update credentials
Need to integrate with a third-party tool that only speaks SMTP

Reach for the HTTP API instead if you:

Want delivery webhooks, idempotency keys, or per-message lookup
Need to send via starter templates with structured variables
Care about response payloads (the SMTP relay returns a 250 OK and that’s it)

SMTP requires a bx_live_… key with the emails:send permission. Preview keys (bx_preview_…) can connect and authenticate via SMTP, but every send is rejected. Sign up free at app.brixus365.com/signup for a live key.

Server config

Setting	Value
Host	`smtp.brixus365.com`
Port	`587` (STARTTLS)
Encryption	STARTTLS — TLS upgraded after initial connection
Authentication	`AUTH LOGIN` or `AUTH PLAIN`
Max message size	32 MB (server announces `SIZE 33554432`)
Extensions	`8BITMIME`, `SMTPUTF8` — non-ASCII recipients and bodies supported

Authentication

The SMTP server uses your existing API key, presented as the password. The username is the literal string apikey:

Username:  apikey
Password:  bx_live_your_key_here

The same bx_live_… key you use for the HTTP API works for SMTP — no separate SMTP credentials. Keep the password secret; rotate via Settings → API Keys in the dashboard.

Make sure your default sender is set before testing SMTP — the relay uses your account’s default sender for the From: header regardless of what you put in MAIL FROM. See Before you send.

Examples

Python

1import os, smtplib, ssl
2from email.message import EmailMessage
3
4msg = EmailMessage()
5msg["From"] = "Acme <hello@yourbrand.com>"
6msg["To"]   = "customer@example.com"
7msg["Subject"] = "Welcome to Acme"
8msg.set_content("Thanks for signing up!")
9
10ctx = ssl.create_default_context()
11with smtplib.SMTP("smtp.brixus365.com", 587, timeout=15) as s:
12  s.starttls(context=ctx)
13  s.login("apikey", os.environ["BRIXUS_API_KEY"])
14  s.send_message(msg)

What preview keys see

If you point an SMTP client at the relay with a bx_preview_… key, the connection succeeds and STARTTLS upgrades cleanly — but AUTH LOGIN fails:

< 220 STARTTLS ready
> AUTH LOGIN
< 535 5.7.8 Authentication credentials invalid

Preview keys are HTTP-only (and starter-templates-only). To send via SMTP, use a bx_live_… key from a free signup or paid plan — see Before you send.

Troubleshooting

Symptom	Cause	Fix
Connection times out on port 587	Outbound port 587 is blocked by your network or cloud provider	Try from a different network. Some hosting providers (Heroku, Railway, Render) block port 25 only — port 587 should be open
`535 Authentication credentials invalid`	Wrong username, revoked key, or a `bx_preview_…` key (those don’t auth via SMTP)	Username is always `apikey`; password is your `bx_live_…` key. Re-copy from Settings → API Keys. Make sure the key has the `emails:send` permission (created via the Send emails (transactional) preset).
`550 5.7.1 No verified sender address configured`	You haven’t set a default sender in the dashboard yet	Open Settings → Email Setup → Sender Addresses and click Set as Default on a verified sender. See Before you send.
`552 Message size exceeds 33554432`	Message body + attachments exceed 32 MB	Reduce attachment size or send the file via a download link instead
`451 Throttled — too many sends per minute`	Per-minute send rate exceeded	Slow down. Server-side throttle protects deliverability

Test locally first. Tools like smtp4dev or swaks let you debug SMTP exchanges without dispatching real email. Once your client speaks valid SMTP locally, repoint it at smtp.brixus365.com.