SMTP: 550 5.7.1 Client host [203.0.113.5] blocked using zen.spamhaus.org
The receiving server looked up your sending IP in a DNS blocklist and found it listed, so it rejected the connection before reading the message. Your content and authentication are irrelevant here; the IP's reputation is the problem. Cloud and residential IP ranges are often listed as a whole, so a brand-new VPS can be blocked on its first email.
Quick fix
Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.
# Which IP does the recipient see? It is in the rejection text
# Check that IP against the list yourself (reverse the octets)
dig +short 5.113.0.203.zen.spamhaus.org
# 127.0.0.2-9: listed. NXDOMAIN: not listed.
# Look up why it is listed and request removal
# https://check.spamhaus.org/
# Residential or dynamic ranges (PBL): do not send direct to MX,
# relay through your provider's smarthost instead, e.g. Postfix:
# relayhost = [smtp.provider.example]:587
# Make sure reverse DNS matches your HELO name
dig +short -x 203.0.113.5
postconf myhostnameHow to diagnose Email errors
SMTP errors follow a strict convention: a 4xx code is temporary and will be retried, a 5xx is permanent and will not. Beyond delivery mechanics, most modern email problems are authentication problems (SPF, DKIM and DMARC alignment) which cause silent filtering into spam rather than an explicit bounce, and therefore need to be checked proactively.
If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.
- Read the full bounce message. The enhanced status code (for example
5.7.1) is far more specific than the three-digit code and usually names the exact policy that rejected the message. - Test the SMTP conversation manually with
swaks --to [email protected] --server smtp.example.com --tls. It shows each command and response. - Verify SPF, DKIM and DMARC records with
dig TXT example.com,dig TXT selector._domainkey.example.comanddig TXT _dmarc.example.com. - Check DMARC alignment, not just presence: the domain in the
From:header must align with the SPF or DKIM domain, or DMARC fails even when both pass individually. - Confirm the sending IP is not on a blocklist and has valid reverse DNS. Missing PTR records cause rejection by many large providers.
Tools worth reaching for
swaksdig TXTopenssl s_client -starttls smtppostfix mail logsDMARC aggregate reports
Authoritative references
Primary documentation for this error, worth reading before applying any fix in production.
Related Email errors
- DKIM: Signature verification failedThe DKIM signature on the email does not match. The DNS public key record may be wrong, or…
- DMARC: dmarc=fail (p=reject) in Authentication-ResultsSPF or DKIM passed on its own but neither aligned with the domain in the From header, and…
- Postfix: 450 4.1.8 Sender address rejected: Domain not foundThe receiving Postfix has reject_unknown_sender_domain enabled and could not find an A or MX…
- Postfix: connect to mx1.example.net[203.0.113.25]:25: Connection timed outYour server never got a TCP connection to the recipient's mail server on port 25, so the…
- Postfix: SASL authentication failure: No worthy mechs foundPostfix tried to log in to the relay and Cyrus SASL could not find a mechanism the relay…
- SMTP: 421 4.7.28 Our system has detected an unusual rate of unsolicited mailGmail is temporarily deferring mail from your IP or domain because recent volume looks like…
- SMTP: 421 Too many connectionsThe mail server is rate limiting connections. Too many simultaneous connections from the same…
- SMTP: 451 4.7.1 Greylisted, please try again laterThe receiving server deliberately rejected a first delivery attempt from an unknown sender…
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Something missing or wrong?
This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.