[pembrook]

Literally all ESPs and SMTP services have super strict anti-spam policies. The problem is it only takes 1 bad send to end up on a blacklist.

The cheapest service will always get the most attempts from spammers. You can try to detect them before they send, but the only way you know for sure that they're a spammer, is after they've already sent spam using your platform.

[jfengel]

I'm a bit surprised that the spammers even bother with it, since it costs money and the conversion rates are near zero. I had the impression that most spammers would rather take over hacked accounts -- not quite zero cost, but pretty low.