MailChimp has a great article, which can explain all you have to know about this problem. For a quick fix you can do one of two things, 1) input your text into a Rich Text Editor, and copy and paste it from there. Or 2) copy your text like normal, but when you past it hold down ⌘ + ⇧ + V (Mac) or CTRL + ⇧ + V (PC).