Ask them how you are supposed to get a visitor's enquiry via a form and reply back to the visitor if the visitor cannot put their own email address in the form. The visitor will have an email address not on your server or a Hotmail or Yahoo etc adress which will be blocked.

Your workaround could be to use an email address of your own domain as the From field and place the visitor's email address in the body of the message. This has to be done via the script after the form is submitted. Then you will have to manually use that address when replying to the visitor.