Mailbox confidence is the mailbox_confidence field: a 0 to 100 estimate, published as a heuristic, that a specific mailbox exists when the SMTP handshake could not say so — because the domain accepts every address, or because the server would not answer. It is computed only in those two cases and is null whenever the handshake settled the mailbox.
What Spaw answers
The estimate starts at 50 and moves on signals already in the answer: an enforcing DMARC policy, an SPF record, an established mail provider and an old registration argue for the mailbox; a gibberish username, a fresh domain and a plus-tag argue against; a username that follows the naming convention seen in the domain's confirmed deliveries is marked up, a name-shaped one that breaks it is marked down; and once a domain has enough reported outcomes its bounce share outweighs the rest. A relay alias is the exception: it stays at 50, and only a high bounce share or a plus-tag lowers it, because the domain's records describe the relay operator, and an alias's name, whether the relay generated it or its owner chose it, says nothing about whether it exists. Every step and its points are published in the risk-score guide, so the figure can be recomputed from the response.
What it does not mean
It is not a probability measured against the world, and the measured tables on the status page exist to show whether it predicts anything. A sending policy can set a floor for it, which is the honest way to use an estimate: decide how sure you need to be, rather than treating 51 as a yes.