An outreach domain is a second domain, like getmargin.example, used only for cold outreach. It has its own mailbox, its own SPF, DKIM and DMARC records, and its own sending reputation.
Cold email attracts some spam complaints even when every message is relevant. Keeping it on a separate domain protects the deliverability of your main domain's password resets, invoices and customer email.
The domain should look like a real company: point it at your site or redirect it there, and ramp sending up slowly.
Example
Margin sends pitches from ana@getmargin.example, which redirects to margin.example, while customer email stays on margin.example.
Putting it into practice
Where to use it: SPF and DMARC record generator, Mailbox warmup calculator and Email deliverability for link outreach. For the longer version, read setting up outreach on Google Workspace.