You cleaned the list, you sent the campaign, and the bounce report is not what you expected. Before blaming the verification, it is worth separating the causes, because they point in different directions.
The first is time. A verdict is true when it is measured. An employee resigns, the mailbox is deleted, and an address verified on Monday is gone by Friday. B2B lists decay faster than consumer ones for exactly this reason, and the older the verification, the more of this you get.
The second is catch-all. Roughly 28% of business addresses sit on domains that accept everything, and those were never confirmed by anyone. If your verifier returned them as valid rather than flagging them, that is where a large share of your bounces came from.
The third is Yahoo and AOL, which accept every probe by policy and bounce afterwards. Any verifier returning confident verdicts there was inferring, and the inference did not hold.
The two that are not about the addresses
Policy rejections. A 550 5.7.1 means the receiving server refused your message over reputation, authentication or a blocklist listing. The mailbox is fine. Your platform files it as a hard bounce and it has nothing to do with list quality.
Rate limiting. A 421 is a temporary failure aimed at your connection. Microsoft in particular does this at volume, and a platform that counts deferrals as bounces will show you a number that describes your send rate.
Separating those two out of a bounce report usually accounts for a surprising share of it, and no amount of cleaning would have prevented any of them.
Where this argument costs us something
The short version
- Re-verify before a large send if the list is more than a few months old.
- Split the bounce report by code before concluding anything about the verification.
- Check whether your verifier labeled catch-all results or quietly called them valid.
Questions people ask
How long is a verification result good for?
A few months for consumer addresses, less for B2B where people change jobs. Re-verify before any large send to an older list.
Should a bounce rate be zero after verification?
No. Catch-all domains, Yahoo, policy rejections and ordinary decay all produce bounces that no verification could have prevented. Under 2% on a cleaned list is a reasonable expectation.