ontains the original message for quick reference and the new message. Forwarding messages: The user agent allows the message to be send to a third party apart from the original sender or the recipients of the copy with or without adding any extra comments. There are two types of mailboxes, called the inbox and outbox. These are actu- ally files with a special format that are created and handled by the user agent (see Figure 29.3). The received mails are kept inside the inbox and all the sent e-mails are kept inside outbox. The messages received in the inbox remain there until the user deletes the messages. User agents can be of two types: Command driven user agents: The command driven user agents were used in the earlier days but are still present as underlying user agents in servers. It performs its task by accepting a one-character command from the keyboard. By typing a predefined character at the command prompt the user can send a reply to the sender of the message and by typing another predefined char- acter at the command prompt the user can send reply to the sender and all the recipients. GUI-based user agents: The GUI (graphical user interface) based user agents contain user interface components like icons, menu bars, windows that allows Internet server server agents agents Figure 29.3 Role of user agents in E2E mail transfer. Fundamentals of EMS, NMS and OSS/BSS the user to interact with the software by using both the keyboard, and the mouse thereby making the services easy to access. There are user agents from multiple vendors that can be used for day-to-day mail communication. Some of the most popular application based user agents include Microsoft Outlook, Mozilla Thunderbird. Some vendors offer Internet mail by hosting mail server on their server and providing the user with Web-based user agent like Gmail, Yahoo Mail, Microsoft MSN Mail, Rediff Mail, and SO on. In Web-based mail, the mails are transferred to the receiving mail server from the sending mail server through SMTP and HTTP is employed in th