The ByteEncoderTypeFor7BitCharsets setting does not take effect for the US ASCII character set after you install the hotfix that is mentioned in Microsoft Knowledge Base article 946641

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: For more information about how to obtain the latest Exchange service pack or update rollup, 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:

• 50220
• 50221
• 50222
• 50225
• 65000
• 20127

For more information, click the following article number to view the article in the Microsoft Knowledge Base:

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.