The User Agent

- A user agent is normally a program that accepts a variety of commands for composing, receiving, and replying to messages, as well as for manipulating mailboxes.

- Some user agents have a fancy menu- or icon-driven interface that requires a mouse, whereas others expect 1- character commands from the keyboard. Some systems are menu- or icon-driven but also have keyboard shortcuts.

Sending E-mail

- To send an e-mail message, a user must provide the message, the destination address, and possibly some other parameters. The message can be produced with a free-standing text editor, a word processing program, or possibly with a specialized text editor built into the user agent.

- The destination address must be in a format that the user agent can deal with. Many user agents expect addresses of the form user@dns-address. In particular, X.400 addresses look radically different from DNS addresses. They are composed of attribute = value pairs separated by slashes, for example,

/C=US/ST=MASSACHUSETTS/L=CAMBRIDGE/PA=360 MEMORIAL DR./CN=KENSMITH/ 

- Most e-mail systems support mailing lists, so that a user can send the same message to a list of people with a single command. If the mailing list is maintained locally, the user agent can just send a separate message to each intended recipient. However, if the list is maintained remotely, then messages will be expanded there.

Reading E-mail
- Typically, when a user agent is started up, it looks at the user's mailbox for incoming e-mail before displaying anything on the screen. Then it may announce the number of messages in the mailbox or display a one-line summary of each one and wait for a command.

                              Figure. An example display of the contents of a mailbox. 

                                                                       

- Each line of the display contains several fields extracted from the envelope or header of the corresponding message. In a simple e-mail system, the choice of fields displayed is built into the program. In a more sophisticated system, the user can specify which fields are to be displayed by providing a user profile, a file describing the display format.

- In this basic example, the first field is the message number. The second field, Flags, can contain a K, meaning that the message is not new but was read previously and kept in the mailbox; an A, meaning that the message has already been answered; and/or an F, meaning that the message has been forwarded to someone else. Other flags are also possible.

- The third field tells how long the message is, and the fourth one tells who sent the message. Since this field is simply extracted from the message, this field may contain first names, full names, initials, login names. Finally, the Subject field gives a brief summary of what the message is about.