550 blockedWhat does 550 blocked mean?
This is a generic permanent rejection with no additional detail from the receiving server. The server is refusing your message but is not disclosing whether the block is due to IP reputation, a DNSBL listing, content filtering, or a manual policy. Generic “550 blocked” messages are common with servers that deliberately obscure block reasons to prevent spammers from gaming their filters. You need to investigate across multiple reputation sources to identify the cause.
Is this a soft or hard bounce?
The block is permanent until resolved. Repeated attempts from a blocked IP worsen reputation scores and can deepen the block.
Common causes
Your IP appears on one or more DNS blocklists. Run a full blocklist check across Spamhaus, Barracuda, Abusix, and SORBS. See blocklist monitoring.
IP reputation services like Sender Score, Talos, or Microsoft SNDS have scored your IP too low for acceptance.
The receiving server’s spam filter flagged your content, links, or sending behavior. No DNSBL listing required for this type of block.
The domain’s administrator manually blocked your IP or sending domain due to past complaints or violations.
How to fix it
Check your IP at MXToolbox against all major blocklists. Identify and delist from any found.
Look up your IP reputation at senderscore.org and talosintelligence.com. Poor scores indicate broader reputation damage.
Clean your list with email verification. Reduce complaint rates. Ensure SPF, DKIM, and DMARC all pass.
If blocklist checks are clean and the block persists, contact the postmaster directly. Many large providers have postmaster contact forms or feedback loop programs.
Provider-specific notes
| Provider | Behavior |
|---|---|
| Generic MTAs | 550 blocked with no further detail is common in custom Postfix, Exim, or milter configurations that deliberately withhold block reason. |
| Corporate mail servers | Enterprise security gateways often return generic blocked messages to prevent information disclosure to spammers. |
| Small domains | Self-hosted mail servers run by individuals sometimes return 550 blocked when adding IPs manually to a local blocklist. |
Related error codes
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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.
About SMTPedia
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