[mjl-]

> this allows those spam checks which will still work to benefit from the normal delivery path.

It's not exactly the same. When a backup MX has accepted the message, it takes responsibility of the message, and will have to send a DSN when it is rejected for being spam. Mox never "delivers" messages to a spam mailbox (it's that behaviour from the bigmail providers I don't like and undermines trust in email!). Mox either accepts a message, or rejects it at the SMTP level. When the backup sends to the primary, and the primary wants to reject, the backup would have to send a DSN to the potential spammer. Not great, and not something we have to do now.

But still, if it's only needed for emergencies, when the primary is down, it probably isn't too problematic. And the backup mx (with primary offline) can always be more strict, requiring dmarc-like alignment before accepting (to prevent backscatter if the primary rejects later on).