A \/24 that looks cheap on the lease listing can be the most expensive block on the market if it carries a blocklist history. Email silently fails, VPN endpoints throw captcha walls, and your engineers spend a week on delisting tickets instead of the project the block was leased for. The check takes twenty minutes before you sign. Here is what to query, what a listing actually breaks, and what each kind of cleanup costs in time.<\/p>\n
No single list governs the Internet; each operator runs its own mix. But four families cover the operational damage. The Spamhaus blocks (SBL and XBL, published via the Spamhaus Project) are the ones with real consequences: a listing there is a strong signal of abuse history and gets honored by a large share of mail operators. Barracuda’s Reputation System is a real-time database of IP addresses with “poor” reputation for sending valid email, manually verified by their team, and it feeds a large installed base of mail appliances. Microsoft’s SNDS and Google’s Postmaster Tools are not blocklists in the classic sense but sender-reputation feedback channels that any serious mail operator should have access to. Finally, the DNSBL aggregators (Barracuda’s lookup, Spamhaus’s lookup, and multi-list checkers) let you query an entire \/24 in one pass instead of address by address.<\/p>\n
| List family<\/th>\n | What it gates<\/th>\n | Typical delisting path<\/th>\n | Cleanup lead time<\/th>\n<\/tr>\n<\/thead>\n |
|---|---|---|---|
| Spamhaus SBL\/XBL<\/td>\n | Mail acceptance at a large share of operators worldwide<\/td>\n | Remediate the source, request review via Spamhaus lookup<\/td>\n | Hours to a few days after fix<\/td>\n<\/tr>\n |
| Barracuda Reputation<\/td>\n | Mail appliances and gateways (Barracuda installed base)<\/td>\n | Removal request via Barracuda lookups page<\/td>\n | Days after remediation<\/td>\n<\/tr>\n |
| Microsoft SNDS \/ Google Postmaster<\/td>\n | Inbox placement at Outlook\/Microsoft and Gmail<\/td>\n | Enroll, build positive sending history<\/td>\n | Weeks (reputation rebuild)<\/td>\n<\/tr>\n |
| Local\/secondary DNSBLs<\/td>\n | Varies; hobbyist to niche ISP filters<\/td>\n | Per-list removal request<\/td>\n | Days, if listed at all<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nWhat actually breaks when your leased \/24 is listed?<\/h2>\nThree distinct failure modes, and only one of them is email. The obvious one: outbound mail from any address on a listed block gets rejected or filed as spam at a large fraction of destinations. The second is web reputation: CDN edge nodes, search crawlers, and security products treat listed space as hostile, which shows up as captcha walls, slower indexing, and broken API calls to providers that filter on reputation feeds. The third is routing itself: some upstreams apply their own admission controls and will push back on accepting a customer announcement for listed space, or will require extra justification before updating their filters. None of these are theoretical; all three show up in the first week of a deployment on dirty space.<\/p>\n How do you screen a \/24 in twenty minutes before signing?<\/h2>\nRun the whole block, not just the addresses you plan to use. Most lessors list blocks by \/24; a single dirty \/32 inside your range is enough to poison deliverability for the neighborhood, because reputation is tracked at both address and prefix granularity. The sequence: query the \/24 network against the major DNSBLs (Spamhaus, Barracuda, and a multi-list checker as a sweep), check the prefix’s routing history on a BGP tool like RIPEstat or bgp.he.net for signs of prior hijacks or announcements from unexpected origins, and run a quick WHOIS to confirm the registered holder matches the lessor you are talking to. Then ask the lessor one direct question: what is your policy if the block gets listed mid-lease? A reputable operator will replace the block or handle the delisting at no charge; silence on that question is itself an answer.<\/p>\n What does ip4.market publish about reputation?<\/h2>\nip4.market scans every listed subnet against major blocklists before listing and monitors continuously, publishing a reputation score (currently 100\/100 on listed blocks) and stating that any block that falls below standard is pulled. That is the kind of contractual answer you want from any lessor, whether the score is published or you have to ask for it. If a lessor cannot state their blocklist screening policy in one sentence, treat the cheap monthly price as the lure it probably is: cleanup time on dirty space will cost more than the discount.<\/p>\n Can you recover from a listing after you have already leased?<\/h2>\nYes, but the clock is the cost. Spamhaus and Barracuda both process removal requests after the underlying cause is fixed, which is the important detail: the delisting request is not the work, the remediation is. If a previous tenant left a spam trap warm, you inherit the cleanup. Mailbox providers are a different story: once your sender reputation at Microsoft or Google has taken the hit, rebuilding it takes weeks of clean sending volume regardless of the blocklist state. This is exactly why the twenty-minute pre-signature check exists: it is the only point in the process where the cost of dirty space is zero.<\/p>\n FAQ<\/h2>\n |