SMTP 554 5.7.1: "rejected due to high probability of spam" and other 554 rejections
What 554 5.7.1 means from Gmail ("Rejected due to high probability of spam"), Microsoft ("5.7.1 Service unavailable, Client host blocked"), Spamhaus listings and content policies — and the fixes: authentication (SPF/DKIM/DMARC), IP reputation, content, and list hygiene.
What 554 means: A permanent rejection after DATA (or at connection) by the receiving server's policy: it decided the message, the sending IP or the domain is not wanted. The mail was not delivered and will not be retried.
Causes, most likely first — and the fix for each
- Gmail "554 5.7.1 Rejected due to high probability of spam" — Google's classifier scored the message as spam: content pattern, sending domain with little history, repetitive notification mail, or many recipients who never engage.
Fix: Authenticate (SPF, DKIM aligned to the From domain, DMARC), warm the domain up slowly, reduce repetitive "X is online"-style notifications, and send only to users who opted in. Check Google Postmaster Tools for the domain's reputation. - Sending IP on a blocklist: "554 5.7.1 Service unavailable; Client host [x.x.x.x] blocked using zen.spamhaus.org" (Microsoft), "sbl-xbl" listings, UCEPROTECT for whole VPS ranges.
Fix: Check the IP with the blacklist tool; delist if it is yours; if it is a cheap VPS range, send through a relay whose IPs are clean instead. - Sending domain listed on Spamhaus DBL, or a link in the body points to a DBL-listed domain.
Fix: Check the domain on check.spamhaus.org; remove listed links; request delisting after fixing the cause. - Microsoft "554 5.7.1 [x.x.x.x] Our system has detected that this message is likely suspicious" or "5.7.1 Unfortunately, messages from [ip] weren't sent. S3140" — Outlook.com reputation.
Fix: Register at Microsoft SNDS / JMRP, fix authentication, lower volume, request mitigation via the Outlook sender support form. - Recipient policy: address does not accept external mail, attachment type banned, message too large.
Fix: Read the full reply text; adjust the message.
How to confirm which one you have
Look at the full bounce text — the part after 554 names the reason and often the blocklist or the postmaster page to visit.
Open the SMTP tester Check SPF / DKIM / DMARC
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Questions about 554
Yes — 5xx codes are permanent failures for that message. The same message re-sent unchanged will be rejected again; fix the cause first.
DKIM aligned to your From domain plus DMARC, a slow volume ramp on a new domain, and cutting repetitive low-value notifications. New domains with sudden bursts are the most common trigger.
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