This can happen even with the best filter, as long as there is some dynamic outside your control. For me, it always flags the newsletter from the Elite Dangerou

This can happen even with the best filter, as long as there is some dynamic outside your control. For me, it always flags the newsletter from the Elite Dangerous-makers (Frontier Games?). And even after setting a filter with the flag to not handle it as spam, it still gets a warning of being a potential flag in the mail-view. That's just the world of our new AI-Overlords. Just cope with it, always check your spam-folder and be nice to the bots.