1. Identify the sending domain
Use the domain associated with the messages that bounced or were rejected. Check your sending platform's configuration and the affected message headers: a service may send from a subdomain such as mail.example.com rather than the root domain example.com. If the visible From domain and the service's return-path or sending domain differ, note both and run a separate lookup for each domain tied to the affected traffic. Enter a domain, not a full email address.
2. Find the outbound IPv4 address, if available
A blocklist result for a domain and one for an IP address answer different questions, so run them as separate lookups. Find the public IPv4 address in your email service's delivery log, sending-IP settings, or the original rejection details. If the service uses a shared or rotating pool, use the address tied to the affected message when it is shown and record that the pool can change. The checker accepts one sending domain or IPv4 address per lookup.
3. Run the lookup and read each provider's result
Use the ReputaPilot blacklist lookup checker with the domain or IPv4 address. It returns a provider-by-provider status for this point-in-time lookup:
- Listed: that provider reported a listing for the checked target.
- Clean: that provider returned a clean result for the checked target.
- Unavailable: the provider did not return a verifiable result. Unavailable does not mean clean.
Save the target, check time, and provider statuses with your incident notes. A later lookup can differ, and this snapshot is not a report of every blocklist.
Open the blacklist lookup checker →4. Gather the bounce or rejection details
Before investigating a result, keep the original non-delivery report or SMTP rejection response. The exact diagnostic text often helps distinguish a blocklist reference from authentication, policy, rate-limit, or mailbox errors. Record:
- The full SMTP response, including its three-digit and enhanced status codes when present, plus the receiving provider's diagnostic text.
- The time and time zone, the receiving mail provider, and whether the failure was temporary or permanent.
- The sending domain, the public outbound IPv4 if known, and the sending service or IP pool involved.
- A message or queue reference, how many messages were affected, and any recent changes to the sender, volume, or sending setup.
Keep the original response intact for your mail administrator or provider. Remove recipient addresses, message content, and credentials before sharing diagnostic material outside your team.
5. Compare the lookup with the rejection
If a provider reports a listing, note the exact domain or IP that was listed and compare it with the rejection's provider, target, and timestamp. If a result is unavailable, treat that provider as unchecked and try again later or consult its current lookup page. A clean result only describes the providers that returned a clean response at that time; it does not establish inbox placement or rule out other causes of rejection.
This page focuses on identification and evidence gathering. For a broader explanation of blocklist types, see the email blocklist guide. If a provider confirms a listing, use the separate blocklist removal guide for operator-specific next steps.