How Outlook applies encoding to plain text e-mail messages

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Article ID: 278134 - View products that this article applies to.
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SUMMARY

This article describes the logic that Microsoft Outlook uses to determine what type of encoding to apply to plain text message bodies that are sent to the Internet. The types of encoding that Outlook can choose from are Base 64, Quoted-Printable, and No Encoding.

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Background

When Microsoft Outlook 2000 is configured for Internet Mail Only (IMO), you can choose which encoding format to apply to all messages that are sent to the Internet.

When Outlook 2000 is configured for Corporate or Workgroup (CW), you cannot choose which encoding format to apply. The transport chooses the encoding format to use.

Each language version of Microsoft Office 2000 has a different default setting for situations where Outlook can control the encoding such as when Microsoft Exchange is not the server.

In the English version of Office 2000, the default is No Encoding, because English text does not contain 8-bit characters. However, in the Japanese version of Office 2000, the default is Base 64 encoding, because Japanese text closely resembles binary data. The most compact encoding for binary data is Base 64.

The versions of Outlook that are listed in the "Applies to" section do not expose the encoding choice as an option. If you use Microsoft Exchange Server, the information store uses its own logic to determine encoding and ignores any setting that you set in Outlook.

Encode Intelligently

The versions of Outlook that are listed in the "Applies to" section encode each plain text body part for which Outlook creates MIME. To do this, Outlook uses the same algorithm that Exchange servers use to send plain text to the Internet. Typically, if 25 percent or more of the message is composed of 8-bit characters, Outlook uses Base 64 encoding. Otherwise, Outlook uses Quoted-Printable encoding.

For more information about encoding issues that occur with Exchange servers, click the following article number to view the article in the Microsoft Knowledge Base:
184718 Text attachments modified for Internet mail

How to Override the Encoding Type by using a Registry Key

Under certain circumstances, you can use a registry setting to force the encoding type.

Important Only advanced users who have to force the encoding type should use this registry setting.

Whether Outlook honors this registry setting depends on what software creates MIME on the user's behalf. When Outlook uses Exchange to send the message, Outlook does not honor this registry setting.

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows

Outlook honors the registry setting if the following conditions are true:
  • The following registry key exists:
    • In Microsoft Office Outlook 2007:
      HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Options\Mail
    • In Microsoft Office Outlook 2003:
      HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Options\Mail
    • In Microsoft Outlook 2002:
      HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Options\Mail
  • The registry key contains a DWORD Value that is named InternetMailTextEncoding.
  • The DWORD value data is 0, 1, 2, or 3.
Outlook acts based on this registry key and value pair as follows:
  • If the value data is 0, Outlook is set to Encode Intelligently.
  • If the value data is 1, Outlook uses Quoted-Printable encoding.
  • If the value data is 2, Outlook uses Base 64 encoding.
  • If the value data is 3, Outlook uses no encoding and leaves 8-bit characters as 8-bit characters.

Properties

Article ID: 278134 - Last Review: January 9, 2007 - Revision: 3.3
APPLIES TO
  • Microsoft Office Outlook 2007
  • Microsoft Office Outlook 2003
  • Microsoft Outlook 2002 Standard Edition
Keywords: 
kbhowto kbreceivemail kbsendmail kbemail kbinfo KB278134

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