Who needs to send 20,000 emails per hour? Other than spamming people, who actually needs to do that?
At 9am on a Monday we send roughly 100k emails via Sendgrid and have very very low spam report rates. Transactional email for even relatively small SaaS companies can get high very quickly.
As much as us in the dev world rely on Slack and try to avoid notification overload, my experience is most people want things as an email.
Other scenario is a business that wants to invite a load of people to something at the same time so they are not sending that many every hour but e.g. tickets for a concert, you want everyone to get the same chance.
Exactly the thing nobody asked for and absolutely deserves to be marked as spam.