How do ISPs merge or separate domain reputations?

ISPs apply heuristics to detect domain relationships:

Shared infrastructure including common IPs, authentication records, or ESP accounts suggests organizational connection.

WHOIS and DNS patterns may reveal common ownership or registration patterns.

Behavioral similarities in content patterns, sending schedules, or recipient overlap suggest related operations.

Explicit signals like organizational domain headers or consistent branding across domains.

When relationships are detected, ISPs may share reputation signals across domains, apply collective scrutiny, or recognize that problems at one domain indicate organizational issues.

Complete separation requires eliminating all detectable connections between domains.

ISPs are investigators connecting dots. Obvious links reveal relationships. Even subtle patterns may be detected. True independence requires leaving no trail between identities.