Although the top-level MIME media types correspond to basic concepts that all users would understand, not all subtypes fall under the obvious media category and those that are associated with specific application software, for instance, may be classified as application types rather than text, image, or audio, despite being widely available over the Internet. Often these data types require a browser plug-in before their contents will be correctly processed when visiting a Web site, or the client browser might ask you to specify which application is associated with file extension or that subtype.

The following table describe most common MIME text types:

MIME defines the following new header fields:

1. MIME-Version, which uses a version number to declare that a message conforms to MIME standard.

2. Content-Type, which can be used to specify the subtype and type of data in the body of a message and to fully specify the encoding of such data. It includes also a subtype option. The seven Content-types specified are:

   1. Text - to represent textual information in a number of character sets.
   2. Image - for transmitting still image (picture) data.
   3. Audio - for transmitting audio or voice data.
   4. Video - for transmitting video or moving image data.
   5. Message - for encapsulating a mail message.
   6. Multipart - to combine several body parts, possibly of different types of data, into a single message.
   7. Application - to transmit application data or binary data.

3. Content-Transfer-Encoding, which specifies how the data is encoded to allow it to pass through mail transports having data or character set limitations.

4. Content-ID (optional), which enables labeling bodies, thus allowing one body to reference another.

5. Content-Description (optional), which enables associating descriptive information with a body.

For anyone who has a requirement to access multiple separate mail accounts that reside on different machines, access to email becomes problematic and many mail systems use the tried and tested POP3 protocol. POP3 client applications download the mail from the server, and store it on the client computer. Most clients store mail in their own format, making it inaccessible by other mail programs, and most machines which run POP3 clients are desktops, which are rarely on 24/7, further reducing access to the mail once it has been downloaded.

The IMAP protocol attempts to remedy some of these problems and the strength of IMAP lies in online and disconnected operation. Unlike POP3, mail is not copied from the server and then deleted - instead, IMAP clients manipulate the mail on the server, and permit access to remote, server hosted mailboxes as though they were local resources.

• An IMAP mail system has a number of immediate advantages for users which are given below:-

• As all mail is stored on the server and changing mail client becomes the work of seconds - all that is required is to configure a new IMAP client with the IMAP account details.

• An IMAP client can easily be configured to view the multiple mailboxes in physically separate servers.

• Multiple IMAP clients can be used by each user and this makes implementing a webmail solution for roaming users a simple task.

• IMAP maintains the message status flags on the server for read, answered, etc.

• IMAP allows the idea of shared folders and this makes it easier to implement generic email accounts for an organisation, and then allow multiple users to access those accounts.

• Many implementations also allow server side filtering of mail and this can be an extremely useful feature when users are accessing their mailboxes through different email clients.