Electronic mail, commonly known as email, allows digital messages to be exchanged between an author and recipients over computer networks like the Internet. Early email systems required both parties to be online simultaneously, but modern email uses a store-and-forward model where messages are sent to and stored on email servers until the recipient connects. An email consists of a message envelope containing sender/recipient addresses, a message header with fields like subject, and the message body. Originally only text, email has evolved to support attachments and non-ASCII character sets.