What it is
Bogons are IP addresses from ranges that are unallocated by IANA or reserved for private use (RFC 1918 space) but which should never appear as source addresses on the public internet. Team Cymru maintains the bogon list to help network operators filter traffic from obviously forged or misconfigured sources. An IP appearing as a bogon is a configuration error, not a spam problem.
How it affects SMTP delivery
MTAs using the Cymru bogon list will reject connections from bogon source addresses. If your mail server's IP is a bogon, it indicates a fundamental network configuration problem: you are either using an unallocated IP range or your traffic is being sourced from an internal address space.
What causes a listing
Your sending IP is in an unallocated, reserved, or private IP range. This is a network misconfiguration, not a reputation issue.
How to get removed
Resolution requires correcting the network configuration to use a legitimate, publicly-routable IP address. The bogon list is policy-based and cannot be petitioned for individual removal.
For the full reference catalog, see the email blocklist directory.
Delisting process in detail
The Team Cymru Bogon list is not a spam blocklist: it enumerates IP address ranges that should never appear as source addresses on the public internet (private ranges, reserved ranges, unallocated space). Delisting happens automatically when the IP allocation status changes: as IANA and the regional registries (ARIN, RIPE, APNIC, AFRINIC, LACNIC) allocate new space, Team Cymru updates the bogon list accordingly. If your sending IP appears on the bogon list, either you are using genuinely reserved space (misconfiguration) or the bogon list is stale relative to a very recent IP allocation.
Prevention practices
Verify that your sending IPs are in allocated, routable space before deploying. Newly-allocated ranges from RIRs sometimes take days or weeks to propagate through all bogon lists; if you receive fresh IP space from a provider, expect a brief period during which some receivers reject connections from the range. Team Cymru updates their bogon list several times per day, but downstream users may cache the data for longer. Never source outbound mail from private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), CGNAT ranges (100.64.0.0/10), or the reserved 240.0.0.0/4 space.
Common listing causes
Legitimate senders on the Cymru bogon list almost always fall into one of two categories: fresh IP allocations from a regional registry that haven’t propagated yet (temporary issue, resolves within days), or misconfigured network address translation that leaks private-range source addresses onto the public internet (real configuration bug requiring fix). If neither applies, verify the IP through the ARIN whois lookup to confirm the current allocation status.
Related blocklists
Bogon monitoring is done by multiple providers: Team Cymru is the most widely used, but Spamhaus DROP and EDROP (Don’t Route Or Peer / Extended DROP) cover related patterns (netblocks fully controlled by spam gangs, hijacked address space). ISPs often integrate all three into their border filtering. For mail administrators, bogon listings are essentially a health check on your provider’s network hygiene rather than a spam signal; if you are on a bogon list, the fix is at the network layer, not the mail layer.
About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
About SMTPedia
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