What It Is
DMARC stands for Domain-based Message Authentication, Reporting and Conformance. It builds on SPF and DKIM. A DMARC record, published in your DNS, sets a policy: let failing messages through and just report them (none), send them to spam (quarantine), or block them (reject). It also asks receiving servers to send you summary reports. Think of it as the instructions you give the doorman about what to do with someone not on the guest list.
Why It Matters for Your Business
Google and Yahoo require DMARC for bulk senders, and having it in place helps any business look trustworthy to mail providers. It also protects your customers. Without DMARC, a scammer can send fake invoices that appear to come from your accounting address. With a strict policy, many of those forged messages never reach anyone.
Good to Know
Do not jump straight to a strict policy. Start with a "none" policy and read the reports for a few weeks to find every legitimate service that sends mail for you, such as your booking system or invoicing software. Fix their SPF and DKIM setup, then move gradually to quarantine and reject. Going strict too early can block your own invoices and receipts.