Why did domain reputation replace IP reputation as the main signal?

Domain reputation became the dominant signal because IP addresses are temporary infrastructure while domains represent permanent brand identity. In the modern email ecosystem, senders switch ESPs, migrate between shared and dedicated IPs, and scale infrastructure constantly. IP reputation proved too volatile and easily abandoned.

A spammer could burn through IP addresses quickly, moving to fresh ones whenever reputation collapsed. By tying reputation to the domain instead, providers ensured that bad actors could not escape their history simply by changing servers.

For legitimate senders, this shift was beneficial. Your domain reputation follows you when you change ESPs, preserving the trust you built rather than forcing you to start over.

IP reputation was like tracking ships by their registration numbers, easily swapped. Domain reputation tracks the flag itself, which a captain cannot simply repaint.