Verifiers use the word in a narrow sense. In the ZapBounce API, invalid is one of four results, next to valid, catch_all and unknown, and it always means a definite no. Unknown is a different thing: the server wouldn't give a straight answer, so nobody knows yet.
An address turns invalid for one of seven reasons, and ZapBounce names the reason on every row. The syntax can be broken, like a missing @ or a space in the middle. The domain may not exist at all, or it exists with no mail server behind it, or it publishes a null MX record that says it takes no mail. At the mailbox level, the server can reply that no such user exists (a 550 code), that the account is disabled, or that the mailbox is full.
A syntax check finds only the first kind. A syntax test passes jane@company.com even after Jane left and IT deleted her mailbox. Only the SMTP step catches that case: your verifier names the address at RCPT TO, the server answers 550, and the connection closes before any message is sent. Our guide on how to check if an email is valid walks through all four tests.
Each invalid address you mail becomes a hard bounce, and bounces are what email platforms measure you on. Amazon SES recommends keeping bounces under 2%, puts an account under review at 5% and may pause sending at 10% (AWS enforcement FAQ, read 24 September 2026). A list with a few percent dead rows can cross that line in one send.
Good addresses also go bad while you're not looking. Hunter tracked verified addresses for eight weeks and found 2.3% had turned invalid in that time (Hunter, 4 November 2025). That's why a list you cleaned in spring isn't clean now.
To find the invalid rows, upload a CSV to bulk verification or paste one address into the free email verifier. ZapBounce returns a result and a reason for every row, keeps your original columns, and charges $5 for 1,000 checks. Unknown results and duplicates are never billed.
Related questions
What does invalid email address mean?
An invalid email address is one that can't receive mail right now. The domain doesn't exist, has no mail server, or the server rejected that mailbox with a permanent 5xx code. Sending to it produces a hard bounce.
Is an invalid email the same as a fake email?
An invalid email is often fake, but a real address turns invalid too when its owner leaves a company and the mailbox is deleted. Disposable addresses are a third case: they can accept mail for an hour and then vanish, so ZapBounce flags them separately from the result.
Should I delete invalid email addresses from my list?
Yes, delete them before your next send, apart from rows marked as a full mailbox. Every invalid address you mail is a hard bounce, and Amazon SES reviews an account at a 5% bounce rate. Re-check the full-mailbox rows after 30 days.
Why does a verifier say unknown instead of invalid?
Unknown means the mail server wouldn't answer the check, often because of greylisting, throttling or a block on probe traffic. Invalid needs a definite no, such as a 550 reply. ZapBounce never bills an unknown result, so that first try costs you nothing.