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

**Code:** `550 5.7.1 Blocked using zen.spamhaus.org` · **Category:** Email / SMTP · **Seen on:** SMTP, Linux

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

## 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?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/email/550-5-7-1-spamhaus-blacklisted)