On a 10,000-address B2B benchmark, about 28% of the addresses sat on catch-all domains. That's the reference point to hold your own list against. If your report lands near it, your list behaves like the benchmark, and more than a quarter of it can't be confirmed at mailbox level by any verifier.
Why so many? Plenty of company mail servers are set to accept every address, so mail to a typo or a former employee still lands somewhere. Every RCPT TO on those servers returns 250. Yahoo does the same on purpose to defeat enumeration, which is why Yahoo-hosted domains effectively always classify as catch-all.
Your own share moves with the mix of the list. A file drawn from a few large companies can sit well above 28% if those companies run accept-all servers, and a list heavy on Yahoo-hosted domains climbs for the reason above. Sort your catch-all results by domain. When a few domains make up most of the segment, one small test send per domain tells you more than any score.
Treat the segment as unconfirmed, not invalid. Send it in a smaller batch on a sending domain you can spare, and remove any address that hard-bounces. Our answer on whether to remove catch-all emails walks through that decision.
ZapBounce proves catch-all with one random-address probe at each domain and reports catch_all as its own result, with its own count at the top of your report. Checks cost $5 per 1,000, so you can measure a 10,000-address list for $25 before you decide how to send it. The catch-all handling guide shows the split in practice.
Related questions
Why do so many of my B2B emails come back as accept-all?
Many company mail servers accept every address, so a verifier cannot confirm single mailboxes there. About 28% of a 10,000-address B2B benchmark sat on such domains, and Yahoo-hosted domains classify as catch-all almost every time.
Does a high catch-all share mean my list is bad?
No. Catch-all describes the mail server, not the person. A share well above the 28% benchmark is worth sorting by domain, but the addresses in it are unconfirmed rather than invalid.
How do I lower the catch-all share of my list?
The share only falls when you remove addresses, because a company's mail server setup is outside your control. Send catch-all addresses in a separate, smaller batch and remove the ones that hard-bounce, and the segment shrinks to the live mailboxes.
How much does it cost to measure my catch-all share?
At ZapBounce a 10,000-address list costs $25 to check, and the catch-all count appears on its own line in the report. Duplicates and unknowns are never billed, and 100 free checks a month cover a first sample.