The biggest mistake was making it trivial to forge email headers. While the spam problem is much better than it used to be, that's because there's a whole lot

The biggest mistake was making it trivial to forge email headers. While the spam problem is much better than it used to be, that's because there's a whole lot of behind-the-scenes (and expensive) infrastructure devoted to combating it. That infrastructure has also made it considerably more difficult to run your own mail server. While you can still do it, there are many hoops to jump through if you want to keep your mail from being dropped on the floor by the first overzealous spam filter it encounters. Unless you're big enough to devote a staff just to that (or you have a lot of free time) it's easier (but more expensive) to just use MailGun, MailChimp, or one of their brethren. I would guess that the annual cost of spam is somewhere in the billions.