Valid vs Invalid vs Risky vs Unknown

Valid — mailbox exists (typical reasonCode mailbox_exists). Safe to treat as deliverable at probe time. Role accounts can still be Valid; they are shared inboxes.

Invalid — cannot receive mail: invalid_syntax, invalid_domain, mx_not_found, smtp_rejected, provider_account_not_found, and similar.

Risky — accepted without mailbox proof: catch_all_detected, provider_non_authoritative_rcpt, mailbox_full, disposable_domain, toxic_domain, possible_spam_trap, and related codes.

Unknown — smtp_timeout, dns_timeout, smtp_blocked, greylisted_persistent, and other unfinished probes. Refunded on live keys.