It then goes a layer deeper, explaining some lower-level concepts. A mail-box is a file, or possibly a directory of files, where incoming messages are stored. A mail user agent, or MUA, is an application run directly by a user. User agents give the message to the transfer agent, who may pass it onto another transfer agent, or possibly many other transfer agents. Users may give messages to transfer agents directly, but this requires some expertise on the part of the user and is only recommended for experts.
Transfer agents are responsible for properly routing messages to their destination.
While their function is hidden from the average user, theirs is by far the most complex part of getting messages from their source to their destination. Delivery Agents Delivery agents are used to place a message into a user's mail-box. When the message arrives at its destination, the final transfer agent will give the message to the appropriate delivery agent, who will add the message to the user's mail-box. A character set is simply a mapping of byte values to characters. Many Asian languages have so many characters that they need multiple bytes to represent them all.