DMARC

You add it as a DNS record. It sets a policy: none (only report), quarantine (send to spam) or reject (block). It can also ask inboxes to send you reports, so you see who sends email in your name. It needs SPF or DKIM in place first.

Gmail requires DMARC for bulk senders. Start with the policy none, read the reports for a few weeks, then move to quarantine. It protects your email deliverability and makes it harder for others to fake your domain. Set it up before you warm up a new domain.