Yes - if the user filling out the form enters an email address that can't be reached you should recieve a failed delivery message to the "From" email address. Check that those failed delivery messages are not caught in a spam filter.
Let us know if this doesn't seam to resolve it, you should recieve those failed delivery messages.
Still haven't seen any returned messages come back. Nothing in my Junk folder. I have confirmed that both email addresses I've used are not valid and should cause a bounceback. test messages to those addresses that I have generated manually out of the "from" email account have returned error messages right away.
Can you please provide unreachable email you used to test so we can try to look up in the logs. Also can you try to use working email in the same situation to double check that this case working. And send both emails directly to