The optimal domain strategy for email marketing after a brand merger is a multifaceted decision influenced by reputation management, tracking requirements, regional considerations, and technical expertise. The key debate centers around using subdomains versus a single domain. Subdomains offer the advantage of isolating reputation, which is crucial when one merging entity has a questionable sending history, allowing for granular tracking and segmentation, and offering a higher degree of control. Conversely, using a single domain may appear simpler but risks one brand's poor practices negatively affecting the entire domain's deliverability. Regardless of the choice, proper authentication (SPF, DKIM, DMARC) is paramount, as is adhering to technical standards and progressively warming up new IP addresses and domains.