I hate that Outlook doesn't send proper emails and always seems to want to put emails inside a winmail.dat attachment. Even though most mail clients/anti-virus/anti-malware products now understand it, I still prefer to turn off the setting in Outlook and let it revert to “standard” formatted emails…
Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Preferences] "DisableTNEF"=dword:00000001
Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\Preferences] "DisableTNEF"=dword:00000001