# 550 5.7.1 Service unavailable; Client host [x.x.x.x] blocked using Spamhaus

- **ID:** `communication/smtp-550-spamhaus-blocked`
- **Domain:** communication
- **Category:** auth_error
- **Error Code:** `550`
- **Verification:** ai_generated
- **Fix Rate:** 78%

## Root Cause

The sending IP address is listed in the Spamhaus Block List (SBL) or Exploits Block List (XBL), causing the receiving MTA to reject the connection as spam.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| Postfix 3.8 | active | — | — |
| Sendmail 8.17 | active | — | — |
| Exim 4.96 | active | — | — |
| Spamhaus SBL v2 | active | — | — |

## Workarounds

1. **Check the IP against Spamhaus using: `dig +short x.x.x.x.zen.spamhaus.org`. If listed, visit https://www.spamhaus.org/lookup/ to request delisting after fixing the spam issue (e.g., securing the server from being an open relay).** (70% success)
   ```
   Check the IP against Spamhaus using: `dig +short x.x.x.x.zen.spamhaus.org`. If listed, visit https://www.spamhaus.org/lookup/ to request delisting after fixing the spam issue (e.g., securing the server from being an open relay).
   ```
2. **Route outgoing mail through a trusted SMTP relay or service (e.g., SendGrid, AWS SES) that has a clean IP reputation. Configure Postfix: `relayhost = [smtp.sendgrid.net]:587` with credentials.** (85% success)
   ```
   Route outgoing mail through a trusted SMTP relay or service (e.g., SendGrid, AWS SES) that has a clean IP reputation. Configure Postfix: `relayhost = [smtp.sendgrid.net]:587` with credentials.
   ```
3. **If the IP is static, contact the ISP or hosting provider to request a new IP address and update DNS records accordingly.** (60% success)
   ```
   If the IP is static, contact the ISP or hosting provider to request a new IP address and update DNS records accordingly.
   ```

## Dead Ends

- **Change the sender email domain to a different one** — Changing the sending domain does not affect the IP reputation; the block is based on the IP address, not the domain. (92% fail)
- **Add SPF and DKIM records to the sending domain** — Adding SPF/DKIM records does not remove the IP from the blocklist; the block is at the IP level and requires delisting. (88% fail)
- **Restart the SMTP server (Postfix/Sendmail)** — Restarting the MTA does not change the IP's blocklist status; the IP remains listed until delisting is requested. (95% fail)