Created SpamProtect. An SMTP Proxy that scanned SMTP traffic for new and unknown email adresses and rewrote the packet data in realtime and placed unsolicited emails in a spam folder. Accepting senders was easy, just mail someone you want to communicate with, and the recipient will be added to an allow list.

Had some nifty features like CIDR blocklists, SMTP Header rewriting (you could impersonate any SMTP server to prevent identification by vulnerability scanners), logging and bandwidth throtteling to reduce the impact of a potential SMTP flood.

This was a commercial product, but we didn't get any buyers for it.