[xs]

Unless you're building an MTA from scratch, I feel like the spam problem is solved by addons the MTA has, like SpamAssassin for Postfix.

[feld]

If I ever get a chance to revamp an email system I will probably use rspamd instead. Those "add-ons" are painful.

https://rspamd.com

https://rspamd.com/rspamd-slides.pdf

[baudehlo]

Check out Haraka, built by an old SpamAssassin developer (ie me). It's all that and has custom anti-spam features like the Karma plugin too. It's in place at some large installs (eg craigslist, who went from 20 postfix servers to just 7 Haraka servers).

[feld]

I'll look into it, thanks!

[seiji]

crm114 isn't actively maintained, but it still works great too.