I have an online form that allows a user to enter the email address they wish to send a message to and I'd like to block certain email addresses, like: gmail, hotmail, yahoo, etc. from using the form. I am using the plugin Contact Form 7 which doesn't have the option to block email addresses and I have not found anything in the forum as a solution. So I thought adding some php would resolve the issue, however I don't even know where to start. Any ideas would be great. Thanks.