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Content Conversion in Exchange Online

Exchange Online
 

Applies to: Exchange Online

Topic Last Modified: 2013-02-25

Content conversion is the process of correctly formatting the underlying code for a message for each recipient. You have a range of options to select in the way you want Microsoft Exchange Online to format messages for external recipients. The decision to perform content conversion on a message depends on the destination and format of the message being processed. In Exchange Online, content conversion includes the Transport Neutral Encapsulation Format (TNEF) conversion options and message encoding options for external recipients. Messages sent to recipients inside the Exchange Online organization don't require this type of content conversion. This topic provides information about message formats and conversion options.

Contents

Exchange Online and Outlook message formats

Content conversion options for external recipients

The following list describes the basic message formats available in Exchange Online and in Microsoft Outlook:

  • Plain text   A plain text message uses only US-ASCII text as described in RFC 2822. The message can't contain different fonts or other text formatting. The following two formats can be used for a plain text message:
    • The message headers and the message body are composed of US-ASCII text. Attachments must be encoded by using Uuencode. Uuencode represents Unix-to-Unix encoding and defines an encoding algorithm to store binary attachments in the body of an email message by using US-ASCII text characters.
    • The message is MIME-encoded with a Content-Type value of text/plain, and a Content-Transfer-Encoding value of 7bit for the text parts of a multipart message. Any message attachments are encoded by using Quoted-printable or Base64 encoding. By default, when you compose and send a plain text message in Outlook, the message is MIME-encoded with a Content-Type value of text/plain.
  • HTML   An HTML message supports text formatting, background images, tables, bullet points, and other graphical elements. By definition, an HTML-formatted message must be MIME-encoded to preserve these formatting elements.
  • Rich text format (RTF)   RTF supports text formatting and other graphical elements. RTF is synonymous with TNEF. TNEF and RTF can be used interchangeably. The rich text message format is completely different from the rich text document format available in Microsoft Word.
    Only Outlook and a few other MAPI email clients understand RTF messages.
  • TNEF   The Transport Neutral Encapsulation Format is a Microsoft-specific format for encapsulating MAPI message properties. A TNEF message contains a plain text version of the message and an attachment that packages the original formatted version of the message. Typically, this attachment is named Winmail.dat. The Winmail.dat attachment includes the following information:
    • Original formatted version of the message, including, for example, fonts, text sizes, and text colors
    • OLE objects, including, for example, embedded pictures or embedded Microsoft Office documents
    • Special Outlook features, including, for example, custom forms, voting buttons, or meeting requests
    • Regular message attachments that were in the original message
    The resulting plain text message can be represented in the following formats:
    • RFC 2822-compliant message composed of only US-ASCII text with a Winmail.dat attachment encoded in Uuencode
    • Multipart MIME-encoded message that has a Winmail.dat attachment
    A MAPI-compliant email client that fully understands TNEF, such as Outlook, processes the Winmail.dat attachment and displays the original message content without ever displaying the Winmail.dat attachment. An email client that doesn't understand TNEF may present a TNEF message in any of the following ways:
    • The plain text version of the message is displayed, and the message contains an attachment named Winmail.dat, Win.dat, or some other generic name such as Attnnnnn.dat or Attnnnnn.eml where the nnnnn placeholder represents a random number.
    • The plain text version of the message is displayed. The TNEF attachment is ignored or removed. The result is a plain text message.
    • Messaging servers that understand TNEF can be configured to remove TNEF attachments from incoming messages. The result is a plain text message. Moreover, some email clients such as Microsoft Outlook Express may not understand TNEF, but recognize and ignore TNEF attachments. The result is a plain text message.
    There are third-party utilities that can help convert Winmail.dat attachments.
    TNEF is understood by all versions of Exchange after Exchange Server version 5.5.
  • Summary Transport Neutral Encapsulation Format (STNEF)   STNEF is equivalent to TNEF in many ways. However, STNEF messages are encoded differently than TNEF messages. STNEF messages are always MIME-encoded and always have a Content-Transfer-Encoding value of Binary. Therefore, there's no plain text representation of the message, and there's no distinct Winmail.dat attachment contained in the body of the message. The whole message is represented by using only binary data. Messages that have a Content-Transfer-Encoding value of Binary can only be transferred between SMTP messaging servers that support and advertise the BINARYMIME and CHUNKING SMTP extensions as defined in RFC 3030. The messages are always transferred between SMTP messaging servers by using the BDAT command, instead of the standard DATA command.
    Exchange Online never sends STNEF messages to external recipients. Only TNEF messages can be sent to recipients outside the organization.

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The content conversion options that you can set in an Exchange Online organization for external recipients can be described in the following categories:

  • TNEF conversion options   These conversion options specify whether TNEF should be preserved or removed from messages that leave the Exchange Online organization.
  • Message encoding options   These options specify message encoding options, such as MIME and non-MIME character sets, message encoding, and attachment formats.

These conversion and encoding options are independent of one another. For example, whether TNEF messages can leave the Exchange Online organization isn't related to the MIME encoding settings or plain text encoding settings of those messages.

You can specify the content conversion at various levels of the Exchange Online organization as described in the following list:

  • Remote domain settings   Remote domains define the settings for outgoing message transfers between the Exchange Online organization and external domains. Even if you don't create remote domain entries for specific domains, there's a predefined remote domain named Default that applies to all remote address spaces (*).
  • Mail user and mail contact settings   Mail users and mail contacts are similar because both have external email addresses and contain information about people outside the Exchange Online organization. The main difference is mail users have user IDs that can be used to log on to the Exchange Online organization.
  • Outlook settings   In Outlook, you can set the message formatting and encoding options described in the following list:
    • Message format   You can set the default message format for all messages. You can override the default message format as you compose a specific message.
    • Internet message format   You can control whether TNEF messages are sent to remote recipients or whether they are first converted to a more compatible format. You can also specify various message encoding options for messages sent to remote recipients. These settings don't apply to messages sent to recipients in the Exchange Online organization.
    • Internet recipient message format   You can control whether TNEF messages are sent to specific recipients or whether they are first converted to a more compatible format. You can set the conversion options for specific contacts in your Contacts folder, and you can override the conversion options for a specific recipient in the To, Cc, or Bcc fields as you compose a message. These conversion options aren't available for recipients in the Exchange Online organization.
    • Internet recipient message encoding options   You can control the MIME or plain text encoding options for specific contacts in your Contacts folder, and you can override the conversion options for a specific recipient in the To, Cc, or Bcc fields as you compose a message. These conversion options aren't available for recipients in the Exchange Online organization.
    • International options   You can control the character sets used in messages.

You can specify the TNEF conversion options at the following levels:

  • Remote domain settings
  • Mail user and mail contact settings
  • Outlook settings, including:
    • Message format
    • Internet message format
    • Internet recipient message format

You can specify the message encoding options at the following levels:

  • Remote domain settings
  • Mail user and mail contact settings
  • Outlook settings, including:
    • Message format
    • Internet message
    • Internet recipient message format
    • Message character set encoding options

For detailed information, see Message Encoding Options.

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