I am having the exact same problem and it is driving me nuts. This has been going on for months, through several updates, and it's making me want to punch Apple in the face. This is some PC type nonsense, is what it is.
All of a sudden I'll open an email and i have to scroll horizontally for weeks to get to the end of each line because mail preview refuses to text wrap it. I cannot stand that there is no option to fix this in preferences or anything, and I can see from other posts in the Apple Support communities that this is NOT a new issue, its been going on with others for MONTHS, since NOVEMBER 2013 it looks like. And I'm no computer programmer, but I know enough about it to know that this sort of fix would take Apple 3 freaking seconds.
COME ON APPLE. I am an attorney and I rely on Apple and Apple mail for business and you continue to COST ME MONEY for how much time I'm having to spend scrolling sideways to read emails and then scroll ALLLL the way back for the next line.
Pathetic, and unacceptable. Fix it Apple. You need to have fixed this like yesterday.