OK, and what did you enter in the form when you tested for the email address?
It is usually not a good idea to actually send the email FROM the entered value because of the high probability of a SPAM check failure. Since the email really isn't coming FROM that person's email server. Try setting the FROM to a valid email address registered on the server sending the email and it may improve the deliver-ability. You can set the Reply-To to the email field from the form separately by clicking the FROM button in the UI and setting it to a separate value.