DNS
What is Subdomain?
A prefixed segment of your root domain (e.g. mail.example.com) used to isolate sending reputation.
A subdomain is a child namespace under a registered root domain, defined in DNS, that can carry its own MX, SPF, DKIM, and DMARC records. Sending reputation accrues partly at the subdomain level, so mail from a subdomain can be evaluated somewhat independently of the root. DMARC still aligns to the organizational domain, so subdomains inherit the parent policy unless one is set explicitly.
Why it matters for cold email
Cold senders almost always send from a dedicated subdomain, often on a separate domain entirely, to quarantine outreach reputation away from the primary domain that carries transactional and employee mail. If a campaign draws complaints or a blocklisting, the damage stays contained to the subdomain instead of poisoning your main corporate domain.
Related terms
Stop guessing at deliverability.
Scheduler Zero handles authentication, warmup, and sender reputation so your cold email lands in the inbox, not the spam folder.