5xx error results in suppression list addition of an email, so future emails won't be delivered (by most ESPs), and not returning MX response would probably be just as bad, or worse (or result in millions/billions of emails being re-queued due to timeouts?)
His solution would result in exponential retry failures baked into most services, which would buy them a few hours, and result in no lost emails, and no suppression list additions.
His solution would result in exponential retry failures baked into most services, which would buy them a few hours, and result in no lost emails, and no suppression list additions.