550 5.7.1 Blocked using zen.spamhaus.org Email / SMTP Origin: Network Last updated 4 Nov 2026

SMTP: 550 5.7.1 Service unavailable; Client host [203.0.113.5] blocked using zen.spamhaus.org

The receiving server checked your IP in a DNS blocklist (Spamhaus, Barracuda, SORBS…) and refused your mail because it is listed.

Exact message you may see Client host blocked using zen.spamhaus.org550 5.7.1 Service unavailable; Client host blocked usingblocked using b.barracudacentral.orglisted at zen.spamhaus.org554 5.7.1 Service unavailable; Client host blocked
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Seen on: SMTP Linux

Meaning

Blocklists list IPs that sent spam, are in residential ranges, or are compromised. Look up the IP on the list's site to see why, fix the cause (compromised account, open relay, malware), then request delisting. Shared hosting and new cloud IPs are frequently listed through no fault of yours.

Common causes

  • Server or an account was compromised and sent spam
  • Open relay or misconfigured form script
  • IP from a residential or dynamic range (PBL)
  • Shared IP with a bad neighbour
  • New cloud IP with a bad history

⚡ Quick fix

  1. Look up your IP at the blocklist site and read the listing reason
  2. Fix the cause, then request delisting
  3. Send through a reputable ESP or SMTP relay while the listing is active

Detailed fix by platform

Linux

  1. Check a list directly: dig +short 5.113.0.203.zen.spamhaus.org (reverse the IP octets); an answer like 127.0.0.x means listed

How to diagnose

  1. List — Which blocklist is named?
  2. Reason — Why is the IP listed?
  3. Fix — Cause removed before delisting?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Network. Errors that are often confused with or caused by this one (click to open):

421 4.7.0 Try again later (rate limited) — SMTP: 421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP address. Mail sent from your IP address has been temporarily rate limited (Email / SMTP)421 4.7.0 Try again l…450 4.7.1 Greylisted, please try again later — SMTP: 450 4.7.1 <user@example.com>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/example.com.html (Email / SMTP)450 4.7.1 Greylisted,…451 4.3.0 Local error in processing — SMTP: 451 4.3.0 Error: queue file write error / 451 Requested action aborted: local error in processing (Email / SMTP)451 4.3.0 Local error…452 4.2.2 Mailbox full / over quota — SMTP: 452 4.2.2 The email account that you tried to reach is over quota (Email / SMTP)452 4.2.2 Mailbox ful…452 4.5.3 Too many recipients — SMTP: 452 4.5.3 Your message has too many recipients / 452 Too many recipients received this hour (Email / SMTP)452 4.5.3 Too many re…550 5.7.25 PTR record required — SMTP: 550-5.7.25 The IP address sending this message does not have a PTR record setup, or the corresponding forward DNS entry does not point to the sending IP (Email / SMTP)550 5.7.25 PTR record…554 5.7.1 Message rejected as spam — SMTP: 554 5.7.1 Message content rejected / Your message looks like spam (spam score exceeded) (Email / SMTP)554 5.7.1 Message rej…554 5.7.1 Service unavailable; client host rejected — SMTP: 554 5.7.1 Service unavailable; Client host [203.0.113.5] blocked / 554 5.7.1 <mail.example.com>: Client host rejected: Access denied (Email / SMTP)554 5.7.1 Service una…550 5.7.1…

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