Just found out that rss2email v3.12.2 uses a User-Agent of "UniversalFeedParser/5.2.1 +https://code.google.com/p/feedparser/" which many, many people block because AI scrapers.

I changed it and see articles from multiple people again…

Thanks to @cks for actually serving me a page saying I'd been blocked, unlike everyone else who just blocked me. I probably should pay more attention to error codes from rss2email.