How does using multiple subdomains affect reputation?

Using multiple subdomains provides strategic isolation for different mail streams but also multiplies the reputation management work required. Each subdomain needs its own SPF, DKIM, and DMARC configuration and must build trust independently.

The benefit is protection. A subdomain used for risky cold outreach can fail without contaminating your high-value transactional domain. Problems stay contained rather than spreading across your entire sending identity.

The cost is complexity. You need to warm each subdomain separately, monitor their individual reputation scores, and ensure authentication stays current across all of them. Neglected subdomains can decay or even become security vulnerabilities.

Running multiple subdomains is like operating a fleet of specialized vessels. Each serves a different route and cargo type, but each also needs its own captain, maintenance crew, and inspection schedule.