This character is a Other Symbol and is commonly used, that is, in no specific script. The character is also known as sealed envelope.
The glyph is not a composition. It has no designated width in East Asian texts. In bidirectional text it acts as Other Neutral. When changing direction it is not mirrored. U+2709 offers a line break opportunity at its position, except in some numeric contexts.
The CLDR project calls this character “envelope” for use in screen reading software. It assigns these additional labels, e.g. for search in emoji pickers: e-mail, email, letter.
This character is designated as an emoji. It will be rendered as monochrome character on conforming platforms. To enable colorful emoji display, you can combine it with Variation Selector-16: ✉️ See the Emojipedia for more details on this character’s emoji properties.
The Wikipedia has the following information about this codepoint:
An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter or card.
Traditional envelopes are made from sheets of paper cut to one of three shapes: a rhombus, a short-arm cross or a kite. These shapes allow the envelope structure to be made by folding the sheet sides around a central rectangular area. In this manner, a rectangle-faced enclosure is formed with an arrangement of four flaps on the reverse side.