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.
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.
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.