Why was an address I believe is active marked "invalid"?

Short answer: invalid means the receiving mail server itself told us the address cannot receive mail — for example, the mailbox does not exist, is disabled, or the domain has no working mail server. This is a definite rejection, not a guess.

What "invalid" is based on

Unlike unknown (where we could not get an answer), invalid is returned only when we have a concrete negative signal. Common reasons:

• mailbox_not_found — the server said this specific mailbox does not exist.
• mailbox_disabled — the mailbox exists but is closed or suspended.
• full_inbox — the mailbox is over quota and rejecting mail.
• host_not_found / no_mx_records — the domain has no reachable mail server at all.
• invalid_syntax — the address is not a valid email format.

Importantly, for providers that hide mailbox existence (Apple iCloud, personal Outlook/Hotmail/Live) we return unknown, not invalid. So an invalid on an ordinary domain reflects a real rejection from that domain's server.

If you are confident the address is active

• It may have been a transient condition. Occasionally a server returns a temporary rejection under load. Re-verifying with a force-refresh request runs a fresh live check that bypasses any cached result.
• Check for typos and formatting. A stray space, a wrong TLD, or an outdated domain will produce invalid.
• The mailbox may genuinely be gone. People close mailboxes and leave companies; an address that worked months ago may no longer exist.

If a force-refresh still returns invalid and you have strong evidence the mailbox is active, contact us with the specific address and we will investigate the exact server response we received.