451 Open relay not allowed: authentication requiredWhat does 451 Open relay not allowed mean?
The receiving server has detected that your session is attempting to relay mail (sending to a recipient outside the server’s accepted domains) without proper authentication. Open relays were a major spam vector in the early 2000s, and modern MTAs are configured to refuse unauthenticated relay by default. The 451 wrapper (vs hard 451 relay not permitted or 550 5.7.1 Relaying denied) is a soft response giving you a chance to authenticate and retry. The fix is to enable SMTP AUTH in your sending application with valid credentials.
Is this a soft or hard bounce?
The server is correctly refusing to act as an open relay. Configure SMTP AUTH with valid credentials and retries will succeed. This is not about IP reputation or content.
Common causes
Session connected and sent mail to an external recipient without performing AUTH. Most common cause.
Sending through port 25 (which typically rejects relay) instead of submission ports 587 or 465.
AUTH attempted but credentials were rejected. Subsequent commands return 451 until session is re-authenticated.
How to fix it
Configure your mail client or application to perform SMTP AUTH with valid credentials. Common settings: AUTH plain, AUTH login, or AUTH cram-md5.
Switch from port 25 (relay-disabled by default) to port 587 (submission with STARTTLS) or 465 (implicit TLS). The submission ports are designed for authenticated relay.
Test with swaks --auth --to test@external.com --server smtp.example.com:587 --tls. If swaks fails, credentials or account state are wrong.
Some SMTP accounts can receive but not send. Verify the account is enabled for outbound relay.
Provider-specific notes
| Setting | Typical value |
|---|---|
| Submission port | 587 (STARTTLS) or 465 (implicit TLS) |
| Authentication method | AUTH PLAIN or AUTH LOGIN, secured by TLS |
| Username | Full email address or account name per server convention |
| Password | Mailbox password or app-specific password |
Related error codes
About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
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