The bit at the end about email deliverability was also interesting:
Notifying our subscribers is another problem... in terms of not ending up on a reputation naughty list or having mail throttled by the receiving server .... Not such a biggy for sending breach notices, but a major problem for people trying to sign into their dashboard who can no longer receive the email with the "magic" link.
And this observation he got from someone:
the strategy I've found to best work with large email delivery is to look at the average number of emails you've sent over the last 30 days each time you want to ramp up, and then increase that volume by around 50% per day until you've worked your way through the queue
This is also known as "warming a domain" in the email world. A large rush of emails from an email server is an indicator of a hack or takeover, so anti-spam software may flag an IP address that surges in activity.
Notifying our subscribers is another problem... in terms of not ending up on a reputation naughty list or having mail throttled by the receiving server .... Not such a biggy for sending breach notices, but a major problem for people trying to sign into their dashboard who can no longer receive the email with the "magic" link.
And this observation he got from someone:
the strategy I've found to best work with large email delivery is to look at the average number of emails you've sent over the last 30 days each time you want to ramp up, and then increase that volume by around 50% per day until you've worked your way through the queue