Glossary: Email
What is DMARC?
A DNS record telling receiving servers what to do with email that fails SPF and DKIM checks, and where to send reports.
What it is
DMARC stands for Domain-based Message Authentication, Reporting and Conformance. It builds on SPF and DKIM. It tells receiving servers what to do if a message claiming to be from you fails those checks, which helps stop criminals sending email in your name.
The three policies
p=none takes no action and only gathers reports. p=quarantine asks receivers to treat failures as suspicious, usually sending them to spam. p=reject asks receivers to refuse them.
How to roll it out safely
Set up SPF and DKIM first. Start with p=none and read the reports to find any genuine services you have missed. Move to quarantine, and then reject, once genuine mail is passing. Starting at reject can block your own email.