Respostas às suas perguntas:
Anexos indesejados, como image001.gif:
O Outlook é muito "inteligente":
How can I prevent an Internet image from being embedded in the message?
After creating a signature with a link to an Internet image, you might experience that the image still gets embedded in the message itself.
Outlook has an option to control to what should happen with linked images; they can either stay linked or Outlook can embed them in the message upon sending.
In Outlook 2003 you can find the option under; Tools-> Options…-> tab Mail Format-> button Internet Format-> disable the option: "When an HTML message contains pictures located on the Internet, send a copy of the pictures instead of the reference to the location."
In Outlook 2007 and Outlook 2010, this option can only be modified via the Registry;
Key Outlook 2007: HKEY_CURRENT_USER\Software\Microsoft\Office.0\Outlook\Options\Mail
Key Outlook 2010: HKEY_CURRENT_USER\Software\Microsoft\Office.0\Outlook\Options\Mail
Value name; Send Pictures With Document Value: 0 (REG_DWORD)
With a value of 1 or if the value name is missing, you’ll include the picture with the message instead of as a link. If the value is set to 0 you’ll send a link.
Note: The value name contains spaces and capital letters; you must include these as well.
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O anexo winmail.dat:
O MS Exchange é muito inteligente:
When an end user sends mail to the Internet from an Exchange Windows or Outlook client, a file attachment called Winmail.dat may be automatically added to the end of the message if the recipient's client cannot receive messages in Rich Text Format (RTF). The Winmail.dat file contains Exchange Server RTF information for the message, and may appear to the recipient as a binary file. It is not useful to non-Exchange Server recipients.
To control whether or not to send messages in RTF, follow the option that best meets your situation: ... Rest can be found on: How to Prevent the Winmail.dat File from Being Sent to Internet Users http://support.microsoft.com/kb/138053
Demorei muito para encontrar essa informação, espero que isso ajude o próximo pesquisador de informações:)