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Introduction

This error means the credentials your mail server or app sent to relay.jetsmtp.net were rejected. The username, the password, or both didn’t match a valid SMTP user on your account. This is a credential problem, not an IP problem. If you’re being rejected because the source IP isn’t on your SMTP user’s allow list, see 535 IP not authorized instead.

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Example error message

host relay.jetsmtp.net [103.173.0.0]
SMTP error from remote mail server after RCPT TO:
535 Authentication failed

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How to fix this error

  1. Confirm the SMTP user still exists. In the dashboard, go to Outbound → SMTP and check that the username you’re authenticating with is still listed and not disabled.
  2. Reset the password. If you can’t be 100% sure the password your server is sending is the one currently on the user, set a fresh password in the dashboard and update the server with the new value. Most authentication failures are stale or mistyped passwords, not anything more exotic.
  3. Check the username format.
    • The SMTP username is the literal string you set when creating the user, not an email address.
    • It must not contain @. If your client requires an email-style login, that’s a sign you copied the wrong field.
  4. Verify the auth mechanism. JetEmail accepts PLAIN and LOGIN. If your client is forcing CRAM-MD5 or another mechanism, switch it to PLAIN (or LOGIN) over a TLS connection.
  5. Check for whitespace and encoding. Trailing spaces, newlines, or invisible characters in stored credentials are a frequent cause. Re-enter the password by hand rather than pasting it.
  6. Retry from the same server. Once you’ve updated the credential on your server, send a test message and watch the response.

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Common causes

  • Stale password after a rotation that wasn’t propagated to every app.
  • Typos in the username or password (including a leading or trailing space).
  • Wrong field used. Some setup forms ask for an email address by mistake; the SMTP username is whatever you set in the dashboard.
  • Deleted SMTP user. Deleting an SMTP user immediately invalidates the credential.
  • Account suspended. If billing is past due or the account is suspended, every authentication attempt will fail. Check the dashboard for any account-level alerts.

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Prevention

  • Keep one SMTP user per app so a rotation only touches the app that needs it. See SMTP users → credential hygiene.
  • Use a secrets manager so production servers always have the current password, never an outdated copy.
  • Name each SMTP user after the app that uses it (wordpress-blog, crm-mailer) so the dashboard list is self-explanatory.

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Still failing?

If credentials are correct, the user is enabled, and the account is in good standing, contact support with the SMTP username (never the password) and the timestamp of a failed attempt. Logs on our side will show whether the request reached us and which check failed.