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.
This is a Received icon. We've made it in iOS style
, first introduced in iOS version 7 and supported in all later releases up until now (at least iOS 11). This style is based on thin two-pixel lines and is optimized for 50x50 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 13,700 icons, all in the same style and quality.