Brevo (Sendinblue) SMTP test — SMTP key, ports, and troubleshooting

Brevo (formerly Sendinblue) provides transactional SMTP relay at smtp-relay.brevo.com on port 587 with STARTTLS. The most common mistake is using the v3 REST API key as the SMTP password — Brevo issues separate credentials for SMTP and REST.

Recommended settings

• Host: smtp-relay.brevo.com
• Port: 587 (STARTTLS) or 465 (implicit TLS)
• Username: your SMTP login email (from Settings → SMTP & API → SMTP)
• Password: an SMTP key from the SMTP keys section (not the v3 API key)
• Auth method: Auto, PLAIN, or LOGIN
• From address: a verified sender or domain in your Brevo account

Finding your SMTP credentials

1. Log in to the Brevo dashboard.
2. Go to Settings (gear icon) → SMTP & API.
3. Under the "SMTP" tab, you will see:
   • Login: your SMTP login email address (this is the username).
   • SMTP keys: a list of generated keys. Click "Generate a new SMTP key" if none exist.
4. Copy the SMTP key value — this is your password.

Do not use the v3 API key from the "API Keys" tab. It looks similar but is for the REST API only and will not authenticate over SMTP.

Testing with SMTP Tester

1. Host: smtp-relay.brevo.com, port 587, security STARTTLS.
2. Username: your SMTP login email from the dashboard.
3. Password: the SMTP key you copied.
4. From: a verified sender address or an address on a verified domain.
5. To: any recipient.
6. Click Run. A 235 Authentication succeeded confirms login.

A 250 OK after DATA means Brevo accepted the message for delivery.

Common errors

535 "Authentication failed" / "Unauthorized" / "Key not found"

• Using the API key instead of the SMTP key: the v3 API key (from SMTP & API → API Keys) does not work for SMTP. Use the SMTP key (from SMTP & API → SMTP → SMTP keys).
• Using the account password: your Brevo login password is not an SMTP credential.
• Wrong username: the SMTP username is the login email shown on the SMTP settings page, not just any email address.
• SMTP key was revoked or regenerated: if you regenerated the key, update all applications using the old one.
• Transactional account inactive: Brevo requires an active transactional plan. If your transactional service is paused or unsubscribed, SMTP auth may fail.

550 "Sender address rejected" / "Not authorized"

The From address is not verified in your Brevo account. Options:

• Verify a single sender: Settings → Senders & IPs → Senders → Add a sender. Click the verification email link.
• Verify a domain: Settings → Senders & IPs → Domains → Add a domain. Add the requested DNS records (DKIM, SPF, DMARC).

Brevo requires sender verification to prevent abuse — even with valid SMTP credentials, unverified senders are rejected.

421 "Too many connections" / rate limit

Brevo limits simultaneous SMTP connections and messages per hour based on your plan:

• Free plan: 300 emails per day.
• Starter: 20K–100K per month depending on tier.
• Business / Enterprise: higher limits.

If you exceed the rate, Brevo returns a 421 temporary error. Retry after a brief pause, or upgrade your plan for higher throughput.

Connection timeout

• Confirm you are using smtp-relay.brevo.com (not old smtp-relay.sendinblue.com which now redirects but may not work in all clients).
• Port 587 is standard. Try port 465 with implicit TLS if 587 is blocked.
• Brevo does not support port 25 for client submission.

Sender verification

Brevo requires at least one of:

• Verified sender address: an individual email address confirmed via a verification link.
• Verified domain: proves ownership via DNS records. Once verified, any address @yourdomain.com can be used as From.

For production use, domain verification is preferred — it unlocks DKIM signing (improving deliverability) and allows any From address on that domain without individual verification.

Legacy hostname

If you see smtp-relay.sendinblue.com in older documentation or configs, it still works (DNS redirects to Brevo infrastructure) but update to smtp-relay.brevo.com for clarity and future-proofing. The credentials are the same.

Brevo SMTP vs Brevo API

Brevo offers two sending interfaces:

• SMTP relay (smtp-relay.brevo.com): works with any SMTP client — WordPress plugins, application frameworks, printers, scanners.
• v3 REST API (api.brevo.com/v3/smtp/email): more features (templates, tags, scheduling), requires the v3 API key and HTTP requests.

Both interfaces deliver through the same infrastructure. Use SMTP when you need plug-and-play compatibility with existing systems; use the API for programmatic control.

WordPress and CMS plugins

WordPress SMTP plugins (WP Mail SMTP, Post SMTP, FluentSMTP) commonly fail with Brevo because users paste the API key into the password field. Always use:

• Host: smtp-relay.brevo.com
• Port: 587
• Encryption: TLS (this means STARTTLS in most plugin UIs)
• Username: SMTP login email from the dashboard
• Password: SMTP key (not API key, not account password)

Test in SMTP Tester first to confirm credentials work before debugging the WordPress plugin.

Security notes

• Keep SMTP keys separate from API keys. If you accidentally expose one, revoke and regenerate it immediately in the dashboard.
• Brevo allows multiple SMTP keys — create one per application so you can revoke individually.
• SMTP Tester redacts credentials from the live transcript.
• Enable webhooks for bounces, spam complaints, and unsubscribes to protect your sender reputation.
• Do not store SMTP keys in version control. Use environment variables or a secrets manager.