I'm guessing that you're using web-based e-mail (i.e., e-mail that you access using your web browser). If you can paste the text into other programs then the problem probably lies in the e-mail web page. Unfortunately since those pages are generated by a remote computer system they can change from one day to the next and that might explain why you're suddenly having the problem.
But I don't use those e-mail providers, so unfortunately I can't really offer much more than that sort of speculation...