This is an HTML part of a message formatted with an unprofessionnal client or directly from a software ill programmed (you can read the culprit name within the raw text source with
Mail it should be within the header field
X-Mailer: unless it is a really very bad software).
Mail doesn't provide any method to display such an ill formatted message as plain text. This is a serious problem since many attack E-mail (most notably phishing attacks) are using this technic to steal private information
or to make you connect on a booby trapped web server.
If you are worried by the global security problem of displaying E-mail messages as HTML, I would advise you to use a more professionnal E-mail client like
In this case, you will be very easily able to display any suspicious E-mail as plain text, but not as raw source, i.e. a humain readable plain text:
View > Message Body As > Plain Text
Another security function of
Thunderbird is worth noting. When you are not sure of the true sender of an E-mail which can very easily be forged, it will enable you to display all headers which will train you to quickly recognize forged
View > Headers > All
Most of the time you will even see the IP address and the program used by the attacker.