acter 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 the transfer of messages to the browser from the Web server. The user sends a message to the Web site to retrieve the mails. The Web site asks the user to specify the user name and password for authentication. The transfer of messages from Web server to the browser happens only after user authentication. The messages are transferred in HTML format from the Web server to the browser that loads the user agent. 29.5 Multipurpose Internet Mail Extensions Multipurpose Internet mail extensions (MIME) is an extension to SMTP that allows non-ASCII data to be sent through SMTP (see Figure 29.4). SMTP has several limitations; such as it cannot be used for languages that are not supported by 7-bit ASCII characters and it cannot be used to send binary files, video, or audio. The MIME acts as a supplementary protocol. The non-ASCII data at the