What Is Subdomain Sending?
TL;DR
Subdomain sending is the practice of sending a mail stream from a subdomain, such as outreach.example.com, rather than from the root domain. The subdomain gets its own SPF record and DKIM keys, and at the major providers it accumulates a sending history that is tracked separately from the root domain used for replies and invoices.
How separate is subdomain reputation really?
Separate, but related rather than independent. Providers track the subdomain in its own right, which is the benefit, while still knowing it belongs to the organisational domain, so a subdomain sustaining real abuse is not a firewall around the parent. Treat it as containment for an ordinary bad patch, not as an escape hatch.
The policy side is explicit. A DMARC record on the organisational domain applies to its subdomains unless the sp tag sets a different subdomain policy, so publishing p=reject at the root and forgetting the subdomain is a common source of surprise rejections.
Which stream should move to a subdomain?
The one whose failure you can afford. Cold outreach and bulk marketing are the candidates, because both generate complaints as a matter of statistics and both can be paused. Transactional mail and person-to-person conversation stay where recipients expect them, on the root domain.
Keep the subdomain recognisable, since the recipient reads it. A prefix that reads like the brand plus a plain word does the job; a string of unrelated characters looks like the throwaway domain of someone who plans to burn it.
Frequently asked questions
Not as a starting balance. A new subdomain begins with no sending history of its own, which means it has to be warmed like any new sender, so the isolation cuts both ways: it protects the root domain and it forgoes the trust the root domain had earned.