Is this email address valid?

Two different things, and people mix them up. The first is shape: a local part, an @, and a domain that exists, with no spaces, no doubled dots and nothing over the length limits. A checker can settle that instantly. The second is whether a mailbox is actually behind the address, and no amount of looking at the string tells you that. The domain's DNS gets you most of the way: an MX record means some server accepts mail for that domain, and no MX and no A record means every message bounces. What this tool reports is exactly that line: well formed, and a domain that takes mail.

Because the mailbox is gone, full, or never existed. A company domain accepts mail for every address under it and only decides at delivery time whether anna@ is a real person; a list that is two years old is full of addresses that pass every syntax check and bounce on contact. The other common case is a typo on a consumer domain: gmai.com is a real domain with a real MX record, so it passes a domain check and still never reaches anyone. That is why the report flags likely misspellings separately and offers the corrected address.

Disposable inboxes are fine for a download and bad for an account you will bill or a list you will send to: the address stops working within hours. Role addresses like info@ and support@ are real and monitored at small companies, and shared, filtered or ignored at large ones. Treat them as a signal, not a verdict: ask for a personal address when you need one, and keep role addresses out of marketing lists, where they attract spam complaints from whoever happens to read the shared inbox. If you send in volume, run your list through bounce codes you already receive before paying a verification service.