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