Earlier quoted context omitted.
Sending an email is absolutely a solved problem. I can send a message to my address in 2020 using a 2003 Fedora Core 1 box without a domain using only what's part of the POSIX standard, and I'll receive it just fine. I just did a few days ago, actually. It even got past my provider's spam filter. I could probably have done it using earlier software than 2003-era mail/mailx, but that was the quickest way I could find…
Sending at scale is the problem. A provider like Mailgun (aka an ESP), is a huge target for abuse. Free and low priced tiers are tricky since they need to draw in customers, but then get heavily abused. tl;dr: Abuse ruins it for everyone. I work on the filtering side of the world so I see a lot of the challenges that even purportedly good actors face. Each provider does better or worse at controlling abuse. These thi…
I'm curious, is that specific to IPv4 or you see the same phenomenon with IPv6 where providers allocate /64 or /48 to customers? Or there simply isn't enough sending volume from IPv6 at the moment to warrant specific filtering?