A fix has now been deployed in all affected instances that should resolve the issue.
once the fix is deployed, the emails that failed to be delivered will be automatically be (re)sent, assuming they didn't reach their attempt threshold (which should not be the case this time)
It appears that we did not correctly evaluate the delivery state of the affected emails. These email were and are not automatically resent. We are looking to do a mass force retry which will re-send them.
We have performed a forced resend on all failed emails.