"Onto" (as one word) is not really commonly used in American English, either. "on to" is used. Let me explain the difference, though. If you got an email, or a task from Jack, you had something you needed to do with it, but you have finished. Now, Jack has something more he needs to do with it. Since it came from Jack originally, you would forward this "back to Jack." If you received an email or a task from someone else, and Jack has never seen this before, but Jack needs to see it, you would "Forward this to Jack" or "Forward on to Jack." Either of those could be used. You could NOT use "Forward this onto Jack" where "onto" is one word. "Onto" means to place something on top of, or similar meanings. It refers to changing position from not being on top of something to being on top of something. For example, "Place the food onto the plate." If you put something onto Jack, then you placed it on top of him ... you might do that with a child, or something if Jack is in the floor, but you wouldn't do that with an email or a task.

Both are correct. I think "onto" is more common in American English but both are acceptable and there is no practical difference in meaning.