This is the Skrzynka pocztowa z listem 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.
The image is of a mailbox with the door opened. The flag is up on the right side of the box. Part of an envelop is showing inside the box. The back of the envelop is showing the seam where the envelop was sealed.