DMARC t= tag

What it means

t= takes y or n (the default). With t=y, a receiver that would quarantine applies none instead, and one that would reject quarantines instead. It may also apply special handling such as From rewriting by mailing lists. The tag does not change reporting, and has no effect on a policy of none.

It replaces the useful part of the retired pct tag: t=y behaves like the old pct=0 and t=n like pct=100. Owners can publish the policy they intend, watch the reports and the effect on intermediaries, and then remove t=y.

Receivers that implement only RFC 7489 ignore t= and apply the policy as written, so during the transition t=y does not guarantee a softer outcome everywhere.