This is the Potwierdzenie odczytu icon for Windows 10. Here we go. Optimized for 32x32 px, these are the simplest icons in the line style. Downloadable formats are PNG and SVG, among others. Icons8 icon pack is unique in providing a huge number of icons designed by the same team of four designers who create the new icons every working day in the same style and with the same quality. Same cool as the Bat Face, huh.
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.