453 you are trying to use me as a relay, but my owner does not allow thisWhat does 453 you are trying to use me as a relay mean?
The 453 response code is non-standard but historically used by Sendmail and older Postfix configurations to refuse open relay attempts with a distinctive human-readable wording. The receiving server detected that your session is trying to send mail to a recipient outside its accepted domains without proper authentication: classic relay attempt that pre-2000 spammers heavily abused. The 453 is technically a soft response (4xx class), giving you a chance to authenticate and retry. Functionally identical to 451 relay not permitted and 451 Open relay not allowed: enable SMTP AUTH or use the correct relay path.
Is this a soft or hard bounce?
The server is correctly refusing to be an open relay. Configure SMTP AUTH or send through a server you are authorized to use as a relay.
Common causes
Session sent to external recipient without performing AUTH. Server refuses to relay.
Sending through a server not authorized to relay to the recipient domain.
Custom application sending direct to recipient MX instead of through your authenticated relay.
How to fix it
Configure your mail client or application with valid credentials. Use submission ports 587 (STARTTLS) or 465 (implicit TLS).
If you control the relay infrastructure, send through your own MTA which then relays out. Most ESPs require this pattern.
Some receivers only accept from specific source IPs. If you cannot relay through their server, send through your own relay to their MX.
For production sending without infrastructure overhead, use an authenticated SMTP relay service.
Provider-specific notes
| Context | Resolution |
|---|---|
| Sending through a public server’s port 25 | Open relay is disabled. Use port 587 with AUTH instead. |
| Custom app sending direct to recipient MX | Configure proper authenticated relay through your own MTA. |
| Trying to use third-party server as relay | Not allowed without explicit relay agreement. Use your own infrastructure. |
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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