550 Sender IP address rejectedWhat does 550 Sender IP address rejected mean?
The receiving server has permanently rejected the connection based on the sending IP address, without disclosing the specific reason. This is a generic IP-level hard rejection: the cause can be a public DNSBL listing, an internal block list at the recipient, a low reputation score, geographic or ASN policy, or a missing PTR. Unlike rejections that explicitly cite a blocklist or policy, this wording deliberately keeps the diagnosis vague (anti-spam strategy: do not give attackers feedback). The fix path is diagnostic: check each likely IP-level cause and address the one that applies.
Is this a soft or hard bounce?
Focus on IP-level signals: DNSBL listings, reverse DNS, sender reputation, and IP geographic context. Authentication and content fixes will not help until the IP issue is resolved.
Common causes
IP on Spamhaus, Barracuda, Abusix, Cloudmark, or similar. Check all major DNSBLs first.
No reverse DNS, or PTR fails FCrDNS validation. See 550 Reverse DNS check failed.
Sender Score, Cisco Talos, or Microsoft SNDS classify the IP as poor or neutral. Recipient threshold rejects below a level.
Some recipients reject all mail from specific countries, ASNs, or hosting providers.
How to fix it
Check Spamhaus, Barracuda, Abusix, Cloudmark, Proofpoint, FortiGuard. Use a multi-lookup tool like multirbl.valli.org.
Run dig -x <sending-ip>. If empty or FCrDNS fails, fix PTR through your IP owner.
Look up at Sender Score, Cisco Talos, Microsoft SNDS. Address weakest signal first.
Residential IPs and many cloud VM ranges cannot recover from this rejection. Switch to an authenticated relay for production sending.
Provider-specific notes
| Diagnostic | First action |
|---|---|
| IP on a public DNSBL | Follow that blocklist’s removal procedure. Some are self-service (PBL), others require manual review. |
| IP has no PTR or fails FCrDNS | Configure PTR through hosting provider, ensure forward A record matches. |
| IP has clean DNS but low reputation | Bounce rate and complaints are the biggest factors. Clean list with email verification. |
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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