I don't have Yahoo mail myself, but here's something you might try. IF you have been creating new paragraphs by hitting the "Enter" button twice after completing the previous paragraph, try hitting Shift+Enter twice instead.
Technically that's creating
line breaks instead of
paragraph breaks, but in a simple email it amounts to the same thing. In fact, I've been using that method routinely myself, since I use Outlook 2007 (offline email client) and it wreaks havoc with formatting depending on my recipients, email service, etc. And the Shift+Enter trick works for me. I use it so much I actually forget about just hitting "Enter."