Mailjet SMTP test — API key and secret key on port 587

Mailjet provides SMTP relay at in-v3.mailjet.com on port 587 with STARTTLS (or port 465 with implicit TLS, or port 25 without encryption). The SMTP username is your Mailjet API key and the password is your secret key — both found on the API keys page of the dashboard.

Recommended settings

• Host: in-v3.mailjet.com
• Port: 587 (STARTTLS, recommended) or 465 (implicit TLS)
• Username: your Mailjet API key
• Password: your Mailjet secret key
• Auth method: Auto, PLAIN, or LOGIN
• From address: must match a validated sender or domain

Finding your credentials

1. Log in to the Mailjet dashboard.
2. Go to Account → API keys (or Settings → API Key Management).
3. You will see your API key (public) and secret key (click "Show" to reveal).
4. The API key is the SMTP username; the secret key is the SMTP password.

These are the same credentials used for the Mailjet REST API (v3/v3.1). Unlike some providers, Mailjet does not issue separate SMTP-only credentials.

Testing with SMTP Tester

1. Host: in-v3.mailjet.com, port 587, security STARTTLS.
2. Username: your API key. Password: your secret key.
3. From: a verified sender address or an address on a verified domain.
4. To: any recipient (or your own address to verify delivery).
5. Click Run. A 235 2.7.0 OK confirms authentication.

A 250 OK after DATA means Mailjet accepted the message.

Common errors

535 "Authentication failed"

• Using your account login password: the SMTP password is the secret key from the API keys page, not your Mailjet login password.
• Swapped username and password: the API key (shorter, public) is the username; the secret key (longer) is the password.
• API key deactivated: if you regenerated credentials, the old pair no longer works.
• Sub-account credentials: if you use Mailjet sub-accounts, each has its own API/secret key pair. Use the credentials from the correct sub-account.

550 "Sender not allowed" / "Sender address rejected"

The From address is not validated in your Mailjet account. Options:

• Add a sender: Account → Sender addresses → Add a new sender. Click the validation email.
• Verify a domain: Account → Sender domains → Add domain. Add SPF and DKIM DNS records.

Mailjet requires sender validation to prevent abuse.

421 "Service temporarily unavailable" / rate limit

Mailjet limits sending rate based on your plan and account reputation. New accounts have a lower initial rate that increases as reputation builds. If throttled, retry after a brief pause.

Connection timeout

• Ensure you are using in-v3.mailjet.com (not legacy hostnames).
• Port 587 is standard. Try 465 if your network blocks 587.
• Port 25 works but sends credentials in the clear if STARTTLS is not enforced — avoid it.

Sender validation

Mailjet requires at least one of:

• Verified sender address: individual email addresses confirmed via a verification link.
• Verified domain: add SPF and DKIM records to your DNS. Once verified, any address on that domain can be used as From.

Domain-level verification also enables DKIM signing, which improves deliverability and is required by many inbox providers.

Sandbox mode

New Mailjet accounts may have sending restricted to verified recipient addresses until the account is reviewed. If sends fail with a policy error after successful AUTH, check your account status in the dashboard — you may need to complete identity verification or contact Mailjet support.

Mailjet SMTP vs REST API

Both interfaces use the same API key / secret key pair:

• SMTP (in-v3.mailjet.com): works with any SMTP client. Good for WordPress plugins, application frameworks, and legacy systems.
• REST API (api.mailjet.com/v3.1/send): more features (templates, personalization, bulk sending, scheduling).

For credential testing and connection debugging, SMTP Tester shows the full SMTP conversation — useful for confirming your setup before integrating with the REST API.

Sending limits

Limits vary by plan:

• Free: 200 emails per day, 6,000 per month.
• Essential / Premium: higher monthly volumes based on tier.
• Burst rate depends on account reputation.

Check your current limits and usage in the Mailjet dashboard under Account → Account overview.

Security notes

• The secret key grants both SMTP and full API access. Treat it as a sensitive credential.
• Do not commit API keys or secret keys to version control. Use environment variables.
• Regenerate credentials if compromised — both the API key and secret key change together.
• SMTP Tester redacts your password (secret key) from the live transcript.
• Consider creating a sub-account with limited permissions for applications that only need sending access.