Google announced a new feature in the Safe Browsing warning set, it will warn users when they will receive an email message from an unauthenticated sender. Gmail users that access the service through their browser or via an Android device will display a question mark over a senders profile photo or user logo if the service cannot authenticate the sender with Sender Policy Framework (SPF) or DKIM. The gradual rollout (potentially longer than 3 days for feature visibility) will impact all Gmail users.”]
Source: https://securityaffairs.co/wordpress/50233/security/safe-browsing-gmail.html