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You install the hotfix that is discussed in Microsoft Knowledge Base article 946641 to set the ByteEncoderTypeFor7BitCharsets key to control the 7-bit transfer encoding method. However, the ByteEncoderTypeFor7BitCharsets setting does not take effect for the US ASCII character set. For example, when you set the value to 2, you expect this value to use Base64 encoding for MIME messages. However, the encoding of MIME messages is still Quoted-Printable (QP).
To resolve this problem, install Update Rollup 7 for Exchange 2007 Service Pack 1.
For more information about Update Rollup 7 for Exchange Server 2007 Service Pack 1, see the following Exchange Help topic:
After you apply this update, Exchange server supports the ByteEncoderTypeFor7BitCharsets option for 7-bit character set US ASCII (20127). The ByteEncoderTypeFor7BitCharsets option supports the following character sets after you install the hotfix:
For more information, click the following article number to view the article in the Microsoft Knowledge Base:
836555 Frequently asked questions about MIME and content conversion in Exchange 2000 Server and in Exchange Server 2003
946641 How to change the method for transfer encoding after you apply Exchange 2007 S946641 to the Exchange 2007-based server that is running the Hub Transport role
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.