Comment by murtza

If you are doing this verification on the client-side, you can restrict form submissions when a user enters a free email address like Gmail or Yahoo.

Here's a post I wrote with an implementation of this approach (specific to Marketo forms, but can be made generic for any form): http://developers.marketo.com/blog/restrict-free-email-domai...

I would also recommend labeling the email address field on the form, "Work Email."