554 5.7.1 — Blocked by spam filter or blocklist
Your mailbox is misconfigured
SMTP error 554 5.7.1 is a hard policy rejection at the end of the transaction: the receiving system judged the message or its sender unacceptable. It most often names a blocklist or a spam verdict, and unlike a 550 it usually arrives after the full message body has been transmitted and scanned.
What the server actually says
554 5.7.1 Service unavailable; Client host [203.0.113.42] blocked using
zen.spamhaus.org; https://www.spamhaus.org/query/ip/203.0.113.42What you see
Sudden, total rejection from one or more providers, frequently naming a blocklist URL in the response. Usually follows a volume spike, a bounce-rate spike, or a compromised mailbox.
Why it happens
- The sending IP is listed on a blocklist such as Spamhaus, SORBS or Barracuda.
- Complaint rate crossed the receiver’s threshold - Gmail acts above roughly 0.3%.
- High bounce rate from an unverified list signalled list-buying behaviour.
- A compromised mailbox or an open relay sent spam from your IP.
- Content matched a known spam signature or linked to an already-flagged domain.
How to fix it
- Read the response for the listing source. The 554 nearly always names the blocklist and a query URL. Check that listing first rather than guessing.
- Fix the cause before requesting delisting. Delisting while the cause is live gets you relisted and makes the next request less likely to succeed.
- Verify every address before sending. Bounce rate is a primary listing trigger. Keep it at or below 1-2%.
- Request delisting, then rebuild slowly. After delisting, resume at a fraction of previous volume and ramp. Returning to full volume immediately re-triggers the filter.
What not to do
- Do not switch domains to escape it. Burning a domain and starting again without fixing the underlying sending behaviour reproduces the problem in weeks, and now across two damaged domains.
- Do not suppress the recipients. They did nothing; the rejection is about your sending identity.
- Do not request delisting repeatedly. Repeat requests without remediation are tracked and slow down future ones.
Frequently asked questions
How long does a blocklist removal take?
Automated listings often clear within 24-48 hours once the behaviour stops. Manual review listings can take days, and some require an explicit request with evidence of remediation.
Difference between 554 5.7.1 and 550 5.7.1?
Both are policy refusals. A 550 typically arrives at the RCPT stage, while a 554 comes after the message body is scanned - so a 554 more often reflects content or a blocklist verdict.
Can warmup fix a blocklisted IP?
Warmup builds reputation; it does not remove a listing. Get delisted first, then use warmup to rebuild standing at a safe volume.
How MailPilot helps
MailPilot classifies this error automatically on every connected mailbox and tells you which side is at fault, so a throttle is never mistaken for a dead recipient and a credential failure never suppresses a healthy address. It also monitors real inbox placement and checks SPF, DKIM and DMARC on every mailbox, which is where most policy rejections start.
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Last reviewed 2026-10-11 by the MailPilot team.