The best solution in this matter is to provide the link to your online form within the email. This way, your end-users will click on the link and have the form opened on a separate page where it’s fully functional. Go to Publish → Link to retrieve the link to your online form.
If you want to read more, follow Campaign Monitor’s article on using forms in email.
In conclusion, it’s not that you can or cannot embed forms into emails. It’s about the fact that online forms don’t work properly or at all on different email platforms (e.g. Yahoo, Gmail, Outlook).