421 Service not available, connection from x.x.x.x refused: DNSBL listedWhat does 421 Connection refused, blocklisted host mean?
The receiving server queried a DNSBL (Spamhaus, Barracuda, Abusix, SORBS, or similar), matched your sending IP, and returned a soft 421 rejection rather than a hard 550. Some MTAs intentionally use 421 for DNSBL matches as a defensive measure: the soft response makes the message queue, giving the sender an opportunity to delist and have retries succeed automatically. The cause is identical to hard DNSBL rejections (your IP is on a public reputation list); the only difference is the recovery path is a bit more graceful. Identify which DNSBL matched, fix the underlying issue, then delist.
Is this a soft or hard bounce?
Despite the soft 421, retries from the queue will keep failing until you address the DNSBL listing. The soft response is just a buffer, not a self-recovery mechanism.
Common causes
XBL, PBL, CSS, or SBL listing on your sending IP. Check check.spamhaus.org.
BRBL match. See 554 5.7.1 Barracuda.
Abusix combined zone listing. Multiple sub-lists possible.
Some receivers query private local DNSBLs not publicly identified.
How to fix it
Most 421 DNSBL rejections explicitly name the matching list. If not, look up at multirbl.valli.org to scan major DNSBLs.
XBL: clean compromised systems. PBL: use authenticated relay. Reputation lists: improve bounce rate and complaints.
Follow the specific DNSBL’s removal procedure. Self-service (PBL) typically resolves in hours; manual review (SBL) takes 24 to 72 hours.
Once delisted, your queue retries will start succeeding automatically without manual intervention.
Provider-specific notes
| DNSBL | Removal speed |
|---|---|
| Spamhaus PBL | Self-service, minutes to hours. |
| Spamhaus XBL/SBL/CSS | Manual review, 24 to 72 hours. |
| Barracuda BRBL | Self-service form, usually under 24 hours. |
| Abusix combined zone | Self-service or manual depending on sub-list. |
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.
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