E-mail absolutely is user-generated content. Section 230 doesn't say anything about "posted publically". Now, there's not quite as much need for it to protect non-public content, because getting people to sue over that is a bit harder, but it really is rightfully protected too.

I don't think that is correct.

Section 230 is a very short section of law, you can read it yourself: https://www.law.cornell.edu/uscode/text/47/230

In any event, it was just an example. Feel free to substitute "HN decides what to show on the homepage" instead.