An unknown verification result means the verifier could not establish whether the mailbox exists: the receiving server refused the probe, deferred it, timed out, or answered in a way that confirms nothing. It is not a judgement about the address. Business domains return unknown far more often than consumer ones because ISP and corporate gateways and some ISPs block verification traffic: in the 2026 corpus 27% of business addresses returned unknown, against 2% of freemail, and one German ISP alone pushed .de lists to 41%. A verifier that reports no unknowns on a business list is guessing on a quarter of it.
Unknown share is reported per job and per domain. A high rate concentrated on a few domains is gateway behaviour; a high rate spread across many domains usually means the verifier's own sending reputation is poor and servers are refusing it. BounceZero refunds credits for unknown results so there is no incentive to label them anything else.
Routing is the response. Unknown leads go to phone, LinkedIn or a warm low-volume sequence on a secondary domain, and are re-checked after a week or two, since gateway behaviour changes. They are never counted as invalid and never sent cold from the primary domain. See unknown results explained.
A 10,000-row list of German SMB contacts verifies at 1,400 invalid, 4,100 unknown, 4,300 valid. The unknown share is driven by one provider's gateway. The 4,300 go to email; the 4,100 go to a phone-first cadence, and a re-check a fortnight later moves 600 of them to valid as the gateway's behaviour shifts.
Verify a list or a single address. 100 free checks a month, unknowns refunded.
Verify Leads - FreeAyoub built BounceZero's 5-stage validation pipeline, its dedicated BGP-announced IP infrastructure, and the Patroni HA PostgreSQL cluster behind every verification. Previously built high-volume email delivery infrastructure. Trained at 1337 Benguerir (École 42 network, 2019). Open-source: bgp_analyzer.