451 relay not permitted!What does 451 relay not permitted mean?
The receiving server is refusing to relay your message to the recipient domain. Relay means forwarding a message to a domain that the server does not directly host. The 451 code (temporary) is less common than 550 for relay errors: some servers issue a soft reject first before permanently blocking. This typically means your sending server is configured to use a mail server that is not authorized to handle mail for the recipient domain, or the recipient domain has not authorized your server to relay for it.
Is this a soft or hard bounce?
Soft bounce in code, but a configuration issue must be resolved. The error repeats on every retry until the relay configuration is fixed. Do not suppress the recipient address.
Common causes
Your MTA is not authorized to relay for the recipient domain: You are sending through an SMTP server that is not configured to accept mail for the destination domain. This is common when using a third-party relay or a misconfigured internal mail server.
Authentication not completed: Some servers require SMTP AUTH before they permit relaying. If your client is not authenticating, the server refuses to relay. See also 554 5.7.1 Relay access denied.
Sending from outside the permitted IP range: The relay server only accepts mail from specific IP addresses. If your sending IP is not on the allowlist, relay is refused.
How to fix it
Step 1: Use the correct outbound SMTP server. Your sending client must use an SMTP server that is authorized to send mail for your domain. Check your email settings for the correct outbound server address.
Step 2: Enable SMTP authentication. Ensure your mail client or application is authenticating with a valid username and password before sending.
Step 3: Check IP allowlists. If you are using a dedicated mail server, confirm your sending IP is added to the relay permissions list.
Step 4: Use an authenticated SMTP relay service. If your current setup does not support proper relay, switching to a dedicated SMTP relay service resolves the issue permanently.
Provider-specific notes
| Provider | Behavior |
|---|---|
| Postfix | Returns 554 relay access denied for hard rejection, 451 relay not permitted for soft. Check relay_domains and mynetworks settings. |
| Exim | Returns 451 or 550 depending on acl_check_rcpt configuration. Enable SMTP AUTH to resolve most cases. |
| Exchange | Returns 550 5.7.54 SMTP Unable to relay for permanent relay blocks. Configure accepted domains and connector permissions. |
Related error codes
554 5.7.1 Relay access denied · 550 5.7.1 Relaying denied · 453 You are trying to use me as a relay · 454 4.7.1 Relay access denied
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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