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DMARC (Domain-based Message Authentication, Reporting & Conformance) is an email authentication standard that lets domain owners publish a policy instructing receiving mail servers how to handle messages that fail SPF or DKIM alignment checks. When a sender’s domain has a DMARC policy, MailChannels reads and enforces it on all inbound messages. The action MailChannels takes depends on the combination of the sender’s published DMARC policy and the spam handling policy you have configured for your protected domain. When the sending domain publishes p=reject, MailChannels blocks unauthorized messages regardless of your local spam handling policy. When the sender uses p=none, DMARC monitoring mode applies and no DMARC-based action is taken. Senders whose messages are rejected due to DMARC will receive a 550 5.7.1 Unauthenticated email is not accepted due to domain's DMARC policy error. See SMTP errors for more detail. To change how your domain handles spam, see Configure spam handling.