Most likely it is the from address.
One way to get around this is to set the from address to be a valid address from your domain.
In the From Address properties, you can set the Return Address to use the Dynamic value entered into the form.
If the email is still landing in the spam folder, we would need to know why it is being treated as spam, usually, the email headers will give an idea of what the issue is. Try examining the email headers to seer if there is a clue there to why the email is being treated as spam.