1. Start Registry Editor. To do so, click Start, click Run, type regedt32.exe in the Open box, and then
   click OK.
2. Locate, and then click the following registry key:

   HKEY_Current_User\Software\Microsoft\Office\11.0\Common\MailSettings

   OR

   HKEY_Current_User\Software\Microsoft\Office\12.0\Common\MailSettings

3. In the right-pane, double-click the WrapLinesDWORD value.
4. Note the value that appears in the Value Data box.
   • If the WrapLines value is set to a non-zero value or is not present (the default setting), plain text lines are wrapped before the e-mail message is encoded. In this situation, you must create the WrapLines value. To do so, follow these steps:
     1. Locate, and then click the following registry key:

        HKEY_Current_User\Software\Microsoft\Office\11.0\Common\MailSettings

        OR

        HKEY_Current_User\Software\Microsoft\Office\12.0\Common\MailSettings

     2. On the Edit menu, click Add Value.
     3. In the Value Name box, type WrapLines.
     4. Click REG_DWORD in the Data Type drop-down list, and then click OK.
     5. In the Data box, type 0, and then click OK.
   • If the WrapLines value is set to 0, no line-wrapping is performed before the e-mail message is encoded.

See http://support.microsoft.com/?id=823921 for more details.