If they send 2 emails per subdomain per year, that could easily be 10s of millions which would make the cost per email measured in thousandths of a cent. And I could see the number of subdomains being larger by a factor of 10, maybe more.

Another angle: someone with IT experience needs to manage the system that seems emails, and other engineers need to integrate other systems with the email reminder system. The time spent on engineering could easily add up to thousands per year, if not tens of thousands.

I’m guessing their figure is based on both running costs and engineering costs.

If your server goes down you’ll miss incoming emails, and IMO residential ISP and power service isn’t reliable enough for your main email address. If mail can’t get delivered people get the impression you gave them a fake email address, which can be more than embarrassing.

It’s the only service I don’t self-hosted, although I do backup my email on my home server. Email protocols don’t tolerate downtime, if your server is temporarily unreachable you won’t get messages and people will probably assume the email address/domain isn’t even valid.

This doesn’t answer OP’s question, but since other people might be interested, MIT also has free graduate level courses. If you choose to pay for a certificate of completion for the courses then they can also count as credits towards a degree at MIT, Harvard, etc.

https://micromasters.mit.edu/