We send a "weekly summary" email of who is absent in your company in the next week.

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.

A large business who gets 20K customer service calls per hour and sends them a CSAT questionnaire? Whether or not you think it is good practice, it happens.

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.

> sends them a CSAT questionnaire

Exactly the thing nobody asked for and absolutely deserves to be marked as spam.

Yup exactly. I just read a thread on Twitter the other day about a company in NYC (can't remember the name) where they sent an email about an event happening the same week. Their customers (double opt-in) never received the email because most of them went to spam or promotions tab due to deliverability issues. It happens.

Many, many businesses have mailing lists with very high numbers (e.g. hundreds of thousands, or even millions of valid, double-opted-in recipients, etc.)...so needing to send out 20k emails per hour is pretty easy.

Plenty of services could use that. Think of any large scale service that does email notifications.

Transactional mail alone can reach that volume easily.

Exactly, think of all the transactional emails companies like Amazon or Stripe send out on an hourly basis.