Opera's email client had them too, I know I was using that around 2000. It was the first time I'd seen the feature, and it was one of the major reasons I used

Opera's email client had them too, I know I was using that around 2000. It was the first time I'd seen the feature, and it was one of the major reasons I used Opera for as long as I did. But Opera, then as now, was far from mainstream...