If you have Outlook 2002 SP1, you can mostly avoid all these bad vibrations by making it display all emails that aren't signed or encrypted (so it is clear they come from a trusted source) in genuine, pure plain text.
Read All Insecure Mail in Plain Text Only in Outlook 2002
To have Outlook 2002 display all insecure messages in plain text:
- Select Run... from the Start menu.
- Type "regedit".
- Hit Enter.
- Go to HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Options\Mail.
- Select Edit | New | DWord from the menu.
- Type "ReadAsPlain".
- Hit Enter.
- Double-click the newly created ReadAsPlain value.
- Enter "1".
- Click OK.
- Close registry editor.

