Apple Mail 7 and older
How can I avoid the display of these poorly formatted HTML messages MS Outlook users are sending me? Well, perhaps the easiest solution would be to ask the sender to simply use plain text for his messages.
By default, Apple Mail displays the “Rich-Text” part of a message, including images, strange formatting, and other oddities. If you prefer Mail instead to just display the plain text of a message by default, open a Terminal and write
defaults write com.apple.mail PreferPlainText -bool TRUE
Restart Mail. Voila, no display problems anymore. (Luckily the e-mail standards require a plain-text version of the contents of an e-mail be present.)
The disadvantage of this solution, of course, is that now any e-mail is displayed as plain text, not only those originating from Outlook/Exchange users. To display the formatted version of a message, if present, use the menu command “View -> Message -> Next Alternative” in Apple Mail.
Note that there is a menu command for switching between the alternative renderings of a mail message.