DMARC Percentage Tag (PCT)

The DMARC Percentage Tag (pct) is used in combination with the DMARC Policy Tag (p) to tell receiving mail servers how to apply the DMARC Policy value to incoming messages that fail DMARC. Basically, the value in the "pct" tag is a percent value (1-100) that tells the receiving mail server, for example, "apply a policy of quarantine to 25% of all messages that fail DMARC". That statement expressed in a DMARC record would look like:

The "pct" tag's primary use is allow a domain owner to gradually test moving a DMARC policy from a policy of p=none to a policy of quarantine or reject (p=quarantine or reject) by testing a small sampling of messages that fail DMARC so that legitimate messages that have delivery issues can be caught early before impacting all legitimate email from that domain.

To get the full benefit of DMARC a policy should eventually be set to either quarantine or reject, but in many cases moving to those settings too fast can cause legitimate email to be accidentally quarantined or rejected. For that very reason the "pct" tag is used to gradually increase the percentage of messages subjected to quarantine or reject and that way any errors detected will have a smaller impact.

One example of a common DMARC record deployment with "p" and "pct" tags would look like the below:

In between each step would be a waiting period of several days (or weeks or months if preferable) during which time DMARC Aggregate Feedback reports can be analyzed to make sure no email delivery issues are detected. After a domain reaches 100% Quarantine, the process would start again with reject increasing from 10% to 100%.