> Admittedly, most residential ISPs block all SMTP traffic, and other email servers are likely to drop it or mark it as spam, but there's no strict requirement for auth.

Source? I've never seen that. Nobody could use their email provider of choice if that was the case.

They don't do DPI, they just look at the destination port. And that's why there's a separate port for submission to mail agents where such auth is expected and thus only outbound mail is typically even attempted to be submitted to. Technically local delivery mail too, e.g. where the From and the To headers are valid and have the same domain.

The 3 most common ISPs in the US are Comcast, Spectrum, and AT&T

Comcast blocks port 25: https://www.xfinity.com/support/articles/email-port-25-no-lo...

AT&T says "port 25 may be blocked from customers with dynamically-assigned Internet Protocol addresses", which is the majority of customers https://about.att.com/sites/broadband/network

What ISP are you using that isn't blocking port 25, and have you never had the misfortune of being stuck with comcast or AT&T as your only option?