Potwierdzenie odczytu and other 65,000 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px. Icons are availble for download in PNG and vector (SVG, PDF, EPS, and more).
There is an image of the back of a mailing envelope. Over the bottom right hand corner of the image there is a circled check mark that seems to be indicating that an action has been completed involving the envelope.