Thanks for your reply. I understand that I can't make them sign up to hushmail or another secure service.
The issue is that It is not only a Contact Form I want to set up but a custom form asking personal questions about mental health issues to help me assess whether they would be suitable for online counselling.
My concern is about how vulnerable that client information would be en route from the website to me. I'm hoping to encrypt the information they enter when they press submit, and keep it secure while it is sent to my hushmail email account, where it is unencrypted for me.
I have paid for an extra Hushmail secure forms service that gives me a line of html code to add to forms to make them secure by encrypting the information. I did this because I read on the WP support pages that the info entered onto a custom form is sent to me in a way that is no more secure than regular email - which I dont feel is safe enough for my clients.
I hope I'm making sense!