What is required
SPF and DKIM for anyone sending at volume, DMARC records, easy one-click unsubscribe, and spam complaint rates kept below a low threshold.
Who is affected
Anyone sending marketing mail or large volumes of transactional mail. Smaller senders are less scrutinised but benefit from the same configuration.
The practical consequence
Misconfigured domains no longer land in spam — increasingly they are rejected outright. The failure is at least visible, which is an improvement.