Valid vs Invalid vs Risky vs Unknown

Valid — recipient accepted under a discriminating probe (reasonCode recipient_accepted_discriminated). This is not proof the mailbox exists, and it is not a delivery guarantee. Send with caution. mailbox_exists is only an authoritative directory confirmation or a sandbox simulation, not the name of a 250. 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.

Confirmed is the only safe-to-send inbox. Mail server accepted it is Valid, send with caution, not confirmed. Accepted — limited verification: the mail server accepted this address, but there is no independent confirming signal. Accepted by receiving server: supporting signs are strong, but there is no mailbox-specific confirmation. Mailbox full means try later, not do not send forever.

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