Messages sent between users of computer systems, the computer systems being used to hold and transport messages. Sender and receiver(s) need not be online at the same time, or even on the same continent, to communicate. Messages can be sent with file attachments, providing a way of exchanging files with other users. Email is an important component of an office automation system.

The originator of a message creates a specially formatted message file by running a mail-sending program. The message may often be entered and modified using the general-purpose editor of the user’s choice. When the message is complete, it is posted to a message transport system, which takes responsibility for delivering the message. This may involve passing the message through a store-and-forward relay system when the sender and receiver are not connected to the same computer. At some later time the message is delivered into the recipient’s incoming ‘mailbox’. The recipient runs a program that retrieves incoming messages, allowing items to be filed, listed, forwarded, replied to, etc. Frequently a single user-interface program is used to send and receive messages both locally and worldwide.

Originally email was performed by using standard text hard copy or CRT terminals. Now systems support the composition and delivery of multimedia mail, which can combine text, graphics, voice, fax, and other forms of information in a single message. Other functions often performed by an email system include verification of a user's identity, expansion of named mailing lists into lists of recipients, and the location of a user on the basis of partial information (directory services).