e 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 sender site is transformed to ASCII data through MIME and are delivered to the client SMTP to be sent through the Internet. The ASCII data are received by the server SMTP and are delivered to MIME to be transformed into original data. Non ASCII code Non ASCII code 7-bit ASCII 7-bit ASCII 7-bit ASCII Figure 29.4 Role of MIME. Mail Communication MIME-Version, Content-Type, Content-Transfer-Encoding, Content-Id, Content-Description are the five headers defined by MIME that can be added to the original SMTP header sections to define the transformation parameters. Let us look into each of these headers: 1. MIME-Version The version of the MIME used, is defined by this header. MIME-Version: 1.1 2. Content-Type => The data type that are used in the body of the message are defined by this header. The type and the subtype of the contents are separated by a slash. The header may contain other parameters depending on the sub- type (see Figure 29.5). Content - Type : < type / subtype; parameters > Subtype Description Plain Unformatted text Mixed Body contains ordered parts of different data types Parallel Body contains parts of different data types Multipart Digest Similar to mixed but default type is