Glossary
Isolated sending subdomain
An isolated sending subdomain is a separate subdomain, with its own mailboxes and DNS records, from which all outbound runs so that nothing about it can affect the company's main domain.
Your main domain carries the mail your business cannot afford to lose: invoices, support replies, contracts. Its reputation was built over years by ordinary correspondence. Outbound is a different activity with a different risk profile, and it should never share that reputation.
So outbound runs from its own subdomain, with 5 to 10 mailboxes, its own SPF, DKIM and DMARC records, and its own warmup. If a campaign goes wrong, the damage stays on the subdomain. The main domain never knows.
The mailboxes on it are function mailboxes, not invented people. A fabricated persona is a deceptive sender identity, and a liability for the client.
Questions
Does a subdomain inherit the main domain's reputation?
Only loosely, and it is not something to rely on. Providers keep reputation per sending domain and mailbox. A subdomain starts closer to zero than to the parent, which is why it is warmed like any new domain.
Can a subdomain hurt the main domain?
Mostly no, which is the point. The isolation is not perfect, so the subdomain is still run carefully, but a bad week there does not land your invoices in spam.
This term belongs to the outbound engineering glossary. The practice is defined on the category page.
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