If you don't like receiving HTML formatted messages, you can easily force them to plain text if you use Outlook 2002-SP1 or later. In older versions of Outlook, you need to use VBA to convert messages to plain text.
In Outlook 2003 and up, go to Tools, Options, Preferences, E-mail options and check the option to read all mail as plain text. You can switch messages to HTML by clicking in the Infobar.
In Outlook 2002 SP1, you'll need to edit the registry or get Attachment Options, donationware from http://www.slovaktech.com/attachmentoptions.htm.
To view messages in HTML, you need to change the registry setting.
The key is:
DWORD of READASPLAIN
Value of 1