I have been having this problem too, seemily started with Mavericks. I was completely stumped, but Kevin's comment above led me to figure out a few more details. The emails exhibiting this problem for me also have a disclaimer at the end of the message. If I reply or forward the message, the message looks OK (correctly word wrapped) while editing, but when I receive a copy of the forwarded or replied mesage, the non-wrap problem appears when viewing the message (and the width of the non-wrap
HOWEVER, as Kevin points out, if I delete the disclaimer, then the sent message is correctly word-wrapped when viewed. Following up on that clue, I discovered that the disclaimer for the emails where I see the problem is in an HTML <PRE> environment (i.e., pre-formatted) with no carriage returns, so it seems that the Mail application is respecting that HTML formatting for the disclaimer, and the length of that <PRE> formatted text defines the width of the window for wrapping all other text in the message. Similarly, if I forward the email and delete only PART of the disclaimer (remember the disclaimer appears to be word-wrapped in the email as you edit, with no hint of the embedded HTML formatting), then when I receive the email the window now "wraps" to the width of the shorted length of the disclaimer text.
So, we need Mail to be able to wrap text to the window width in ALL cases, either by ignoring the HTML <PRE> environment or disable HTML . Even just showoing a message in plaintext is fine, but it sounds like this option no longer is available in Mavericks:
https://discussions.apple.com/thread/5485133
https://discussions.apple.com/thread/5473035