521 <hostname> does not accept mail from your IPWhat does 521 connection rejected mean?
The 521 response code is non-standard in RFC 5321 but is widely used by MTAs (notably Postfix) to signal I do not accept mail: the receiving server has refused the connection at the SMTP greeting stage, before any envelope or message data was transmitted. The cause is almost always IP-level: DNSBL listing, internal block list, geographic policy, or PTR validation failure at greeting. The 521 differs from 554 connection-refused variants in that it specifically signals this MX does not accept mail at all from your context rather than refused for policy. Common at servers configured to reject before any SMTP transaction starts.
Is this a soft or hard bounce?
The rejection happens before any envelope is sent. Diagnostics focus on IP-level signals: DNSBLs, PTR, reputation, and geographic policy.
Common causes
Receiver checks DNSBLs immediately on connection. Match returns 521.
Some receivers reject at greeting if PTR is missing. See 550 client rejected: no rDNS.
Receiver rejects all mail from specific countries, ASNs, or hosting providers.
Recipient-internal block list checked at connection. Removal requires direct postmaster contact.
How to fix it
Look up at Spamhaus, Barracuda, Abusix, Cloudmark, Proofpoint, FortiGuard. Address any listing.
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 weak signals.
If reputation and DNS are clean, the cause is likely internal block list or geographic policy. Reach out to recipient admin.
Provider-specific notes
| Diagnostic | Action |
|---|---|
| IP on public DNSBL | Follow that blocklist’s removal procedure. |
| No PTR or FCrDNS fails | Configure reverse DNS with hosting provider. |
| Clean DNS and reputation | Contact recipient postmaster for policy review. |
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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