Or any domain for which you can read an email sent to an inbox. I remember a few years ago an attack where the attacker would read email because a ticket would be created for incoming emails, and he could guess the next ticket ID to read it. A lot of platform that aren't email providers still allow emails in (e.g. GitHub, GitLab). This looks like a rather widely-applicable attack.

edit: I was thinking about this: https://news.ycombinator.com/item?id=41818459