I can’t remember the site, now, but I literally couldn’t log into one this week because the email never arrived.

Well, email allows you to solve that issue by self-hosting. But what you can't solve is that if you do self-host, gmail will drop your emails to spam or just discard them completely, just because it feels like it, even if you do the whole dance with DMARC and have used the domain for a good few years. It's frustrating as shit.