What is email address verification?
Email address verification is checking whether an address can receive mail before you send to it. Anyone can check the format, the domain’s mail servers and obvious typos. Whether the mailbox exists can only be asked of the receiving server over SMTP, and a catch-all domain says yes to every address.
Reading the result
Everything runs in your browser except one MX lookup, which receives the domain and nothing else: the address never leaves the page. You see whether it is well formed, whether the part before the @ is an RFC 2142 role name such as info or postmaster, whether the domain is a known misspelling, and which servers take its mail. Whether the mailbox exists is not checked, because Cloudflare Workers, where this site runs, cannot open connections on port 25.
Common problems it finds
A misspelled provider domain
gmial.com and hotmial.com look right at a glance. The tool suggests the domain from a short, hand-written list of misspellings of large mailbox providers, and leaves the correction to you.
A role address
Names like info, sales and postmaster are defined in RFC 2142 for functions, not people. They often reach a shared inbox, where nobody signed up as an individual.
A domain that takes no mail
A null MX ("0 .", RFC 7505) says the domain accepts no mail at all, so sending to it can only bounce. A domain with no MX record falls back to its own address record, which this check does not read.
Expecting a mailbox check
Tools that promise one connect to the receiving server on port 25 and ask. This site cannot, and a catch-all server would say yes anyway. Your own bounces are the reliable answer: a 5.1.1 means the mailbox does not exist.