How to isolate domains by brand or department?

Effective domain isolation requires separation at multiple levels:

Separate root domains rather than subdomains for brands requiring maximum isolation. Subdomains share some parent association.

Independent authentication with unique SPF records, separate DKIM keys, and individual DMARC policies for each domain.

Dedicated IPs for high-volume brands, preventing shared infrastructure contamination.

Separate ESP accounts or configurations to prevent cross-contamination through shared suppression lists or settings.

Distinct link tracking domains since shared tracking domains can associate otherwise-separate brands.

Complete isolation is like operating truly independent shipping companies. They may share common ownership, but their vessels, crews, and port relationships are entirely separate.