What’s the role of ISPs in filtering?

Internet Service Providers and mailbox providers are the gatekeepers of email delivery. Gmail, Microsoft, Yahoo, and regional ISPs all operate spam filters that protect their users from unwanted and malicious mail.

These providers make autonomous filtering decisions based on their own data and policies. They track sender reputation internally, monitor complaint rates from their users, and apply machine learning models trained on their specific mail flows. What works at one provider may not work at another.

ISPs also participate in industry initiatives like Feedback Loops, which notify senders when recipients mark their mail as spam. This collaboration helps legitimate senders maintain good standing while isolating bad actors.

Each ISP is a sovereign harbor with its own rules. Senders must earn trust at every port, not just one.