What it means
A secondary domain is a close variant of your brand's domain, bought only for outbound email, such as getacme.com for a company at acme.com. Cold email puts sender reputation at risk, because bounces and spam complaints count against the domain that sent them.
Sending from secondary domains keeps that risk away from your main domain, which handles invoices, support and everyday email. Each secondary domain usually redirects to your main website, so a prospect who checks it lands in the right place.
How we use it
Cold email always runs on secondary domains, never a client's main one. A reputation problem there never touches the inbox their clients write to. Each domain carries a few inboxes, and every one passes SPF, DKIM and DMARC before warm-up starts.
We read the client's site first, so the lookalike domains we suggest read as legitimate, and nothing is bought until the cost is confirmed. Spreading the volume over several domains and inboxes also means one tired inbox can be rested without stopping a campaign.