Most engineering and IT teams set up SPF ( v=spf1 ) and DMARC ( v=DMARC1 ) once during domain onboarding, see a green checkmark in their mail console, and never look at it again—until transactional emails, invoices, or password resets start silently landing in spam under the Google, Yahoo, and Microsoft bulk-sender enforcement mandates or failing a UK Cyber Essentials v3.3 ("Danzell") / EU NIS2 audit.

Here are the three silent failure modes we see across hundreds of production domains at FaultyPacket Security Lab , along with how to test and fix them without buying a $600/year recurring SaaS subscription.

1.…