Mails message contents not 'wrapping' (i.e. running off edge of box)

Solution 1:

Looks like an HTML email has some CSS settings off the mark. Sender's fault. There is a little you can do. Hit altcmdU and see it in an even worse format but at least the text lines would break way before the width ends.

The shortcut corresponds to View menu → Message → Raw Source.

Solution 2:

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).

Unfortunatly, 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 Thunderbird:

https://www.thunderbird.net/en-US/

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 From: and To: headers:

View > Headers > All

Most of the time you will even see the IP address and the program used by the attacker.