$ man secondary-domains

Secondary Domains

Separate domains used exclusively for outbound email. Not your primary company domain. Variations like "acme-team.com" or "tryacme.io" that protect your main brand reputation if something goes wrong.

If you send cold email from your primary domain and deliverability tanks, your entire company's email reputation goes with it. Sales emails, support emails, internal comms — all damaged. Secondary domains create a firewall. If one burns, you rotate it out. Your primary domain stays clean. I learned this the hard way when I was an SDR sending from the company's main domain with zero warmup. Half my emails hit spam. IT had to intervene.

Every partner campaign gets dedicated secondary domains. I register variations of the brand — close enough to be recognizable, different enough to protect the primary. Each domain gets its own DNS records (SPF, DKIM, DMARC), its own warmup cycle, and its own sending limits. If a domain's reputation drops, I pull it from rotation and swap in a spare. The primary domain never touches outbound.