# 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

**Code:** `554 5.7.1 Service unavailable; client host rejected` · **Category:** Email / SMTP · **Seen on:** SMTP

The server refused the connection from your IP because it is blocked by policy, not in the allowed list, or listed on a blacklist.

## Meaning

Unlike a content rejection, 554 5.7.1 on connect refers to the connecting host. The remote admin has an IP allow/deny list, geo block, or uses a DNSBL. Check blocklists first, then contact the postmaster of the remote domain with your IP and the timestamp from the bounce.

## Common causes

- Your IP is on a DNSBL or the remote admin's deny list
- Server accepts mail only from specific IPs
- Missing or mismatched reverse DNS
- Country/ASN-level block
- Previous spam from the IP range

## Quick fix

1. Check your IP on blocklists (mxtoolbox.com/blacklists)
2. Fix reverse DNS, SPF and DKIM
3. Contact the recipient's postmaster with the bounce details

## Detailed fix by platform

### Linux

1. `host 203.0.113.5` (PTR must exist and match the HELO name) and `dig +short TXT example.com` for SPF

## How to diagnose

1. **IP** — Which IP is blocked?
2. **Lists** — Listed anywhere?
3. **DNS** — PTR and SPF correct?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/email/554-5-7-1-client-host-blocked)