How to check an IP blocklist for email delivery problems
Find the actual outbound IPv4 in a delivery report, interpret blocklist results and decide who can resolve the issue.
Separate rejection from spam placement
A delivery report may show a temporary 4xx or permanent 5xx SMTP response. Save the text as well as the code. A nonexistent mailbox is not fixed by delisting an IP. If the message landed in spam, it was accepted, so inspect a received sample and look for patterns across recipient providers.
Find the actual sending IP
Your site’s CDN, inbound mail server and outbound provider can use different addresses. Start with the connecting IP in the recipient’s error or your sender’s delivery log. Trusted Received headers on a delivered test can also help. If the address belongs to a shared sending pool, ask the provider to investigate it.
Interpret the check and address the cause
Astrotor checks SpamCop and PSBL for public IPv4 addresses. A timeout is different from a clean result, and neither list represents a mailbox provider’s private reputation system. Compare any listing with the exact bounce. Before seeking delisting, investigate sudden sending spikes, a compromised mailbox, a misbehaving app or poor list hygiene.
If the address is clear
Check SPF, DKIM and DMARC. If you run your own mail server, confirm reverse DNS with the provider. Give support a timestamp, recipient domain, outgoing IP and exact error so they can locate the delivery attempt.
IP blacklist check
Enter the public IPv4 from the delivery error, which may differ from your website’s IP.
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