This is a rectangle depicting an envelope. There is a triangle inside the rectangle pointing downwards, depicting the flap of the envelope.
This New Post icon is made in Material Design style
developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 6,200 icons, all in the same style and quality.