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Internet Mail Formats

The Internet Mail Service formats outbound messages according to RFC 822. This RFC assumes that the message body consists of 7-bit ASCII characters and that the lines in the message body do not exceed 1,000 characters.

However, many people now send messages that contain rich text, such as bold and italic formatting, documents and spreadsheet attachments, images, audio, and video. Because most of these types of data are in binary format, specific mechanisms for conveying this data in messages are required.

The Internet Mail Service supports the following mechanisms for sending binary data in Internet mail.

Multipurpose Internet Mail Extensions (MIME)

MIME is the standard for exchanging multimedia messages on the Internet. MIME describes the contents of a particular type of data so that the receiving system can display the data accordingly. For example, suppose that a message includes an attachment created in Microsoft Word. When the message is sent, the Internet Mail Service can tag the data as being of type application/msword so that the recipient can view the contents of the attachment by using Microsoft Word.

MIME specifies seven major content types. Each content type can have many subtypes to further specify the content of data.

Primary Type Description
Text Content that is primarily textual and probably can be read in raw form
Application Content that doesn't fit into any of the other categories
Multipart Message made up of multiple parts, such as both text and an application
Image Pictures
Audio Sounds
Video Moving images
Message Common when dealing with embedded messages or delivery reports such as NDRs or Read Receipts

File attachments from an Apple Macintosh® computer will be mapped to MIME-standard body parts so that MIME-compliant clients and gateways can decode these attachments. Additionally, the Internet Mail Service maps any incoming body parts to Windows or Macintosh file types.

Uuencode and Uudecode

Uuencode and uudecode are tools that prepare binary files for transport between UNIX systems. Uuencode is used extensively to encode non-7-bit data in messages. When the message is received at the destination, the encoded data must be decoded by uudecode. The Internet Mail Service automatically encodes data in outbound messages and decodes parts in incoming messages.

Microsoft Exchange Server Rich Text Formatting

Although Internet mail provides extremely rich formatting capabilities for communicating data, certain types of information are not communicated well, such as positional information for attachments and OLE objects as attachments. These limitations can be overcome when communicating with other Messaging Application Programming Interface (MAPI)-based systems by sending Microsoft Exchange Server rich text formatting information in Internet mail. The receiving system, if MAPI-capable, can reconstruct the message as if it were sent between two Microsoft Exchange Server recipients on the same server, preserving all the positional information and data.

Either MIME or uuencode can be used when sending Microsoft Exchange Server rich text formatting in Internet mail.

Choosing a Formatting Method

When configuring the Internet Mail Service, you can choose whether attachments should be encoded through MIME or uuencode by default, and whether Microsoft Exchange Server rich text formatting should be used. These settings can also be specified on a per e-mail domain basis. When composing a message, users can override the option to encode attachments through MIME or uuencode. To override this option, Send Options for Internet is used.

Note   Users can also indicate whether Microsoft Exchange Server rich text formatting should be sent on a per-recipient basis. The administrator can also indicate whether a custom recipient should be sent Microsoft Exchange Server rich text formatting. The default or per e-mail domain setting for rich text formatting should be set to User for this to work.