Customize email validation

The plugin seems to work in this version of wordpress also. My question is how can i chaange the validation rules for the email field. I need that the email entered by the user in the contact us form should be a corporate/company email, ie i want to create a blacklist for domains like gmail,hotmail, yahoo etc.. and implement the blacklist rules on the field.
pls can you help me with this?

http://ww.wp.xz.cn/extend/plugins/visual-form-builder/