The mail server does not always tell the truth
A verifier learns about a mailbox by asking the receiving server during an SMTP session. That server can decline to answer by greylisting with a temporary code, can block probing entirely, or can accept every address because it is configured as a catch-all. In each case the probe returns no reliable information about the mailbox.
These are not rare edge cases. Catch-all configuration is common on business domains, and greylisting is standard on many corporate mail gateways. A provider that reports every such case as deliverable will look more accurate on paper and bounce more in practice.
- Greylisting: the server says try later and the result is unknown until a retry.
- Blocked probes: only syntax, DNS, and MX checks complete.
- Catch-all: every address is accepted, so acceptance proves nothing.
- Rate limiting: the server starts refusing after a number of checks from one source.
Mailboxes change after the check
A deliverable result is true at the moment it was produced. Mailboxes are deleted when people leave companies, fill up when they stop being read, and get converted to aliases or group addresses. Providers also reclaim abandoned accounts and some turn them into spam traps. The longer the gap between verification and sending, the more of these changes accumulate.
This is why re-verification before a campaign matters more than a perfect score on the original check, and why VeriDock makes re-verifying the same address free within 30 days.
A verified address is not an inbox placement
Even when the mailbox exists and the server accepts the message, delivery is not finished. Spam filters evaluate the sender's reputation, SPF and DKIM alignment, DMARC policy, message content, and past engagement. A message can be accepted with a 250 reply and then dropped into spam, quarantined by a corporate gateway, or discarded silently by a filter the recipient never sees.
Verification governs one input to that decision: whether you are sending to real addresses. It cannot govern the others.
Verification reduces bounces. Reputation, authentication, and content decide inbox placement.
How to evaluate accuracy claims
A single percentage without a method behind it is marketing. Ask what the ground truth was, how catch-all and unknown results were counted, whether the test set included greylisting domains, and whether the numbers came from a reproducible run. A provider that returns a large share of risky and unknown results on a difficult list may be more trustworthy than one that reports everything as deliverable.
The most reliable evaluation is your own. Verify a sample of your real list, send to it, and compare the bounce log against the predicted statuses. VeriDock's free tier covers 250 checks a month with no card, which is enough to run this experiment before committing.
Key takeaways
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Frequently asked questions
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