What happens when DMARC fails?

When DMARC fails the receiving server applies the policy defined in your DMARC record. With p=none the message is delivered normally but logged in your reports. With p=quarantine the message is placed in spam. With p=reject the message is blocked entirely. DMARC failure does not always indicate an attack. It often means a legitimate sender is misconfigured. Reports help you distinguish between misconfiguration and spoofing.