What is email verification?

Verification runs a series of checks that get progressively more expensive: whether the address is shaped correctly, whether the domain exists and publishes a mail server, and whether that server accepts mail for the specific mailbox.

The last check is the one that matters and the one that fails most often, because many mail servers deliberately decline to answer. A server configured to accept everything addressed to the domain will say yes to any mailbox, real or not, which is the single largest source of false confidence in this industry.

So a verification result is not a yes or no. It is a yes, a no, or an honest admission that the server will not say, and a tool that collapses the third case into the first is selling you a bounce rate you have not measured yet.