Email not valid

I have had many clients complain that when they try to register on our website it shows the error "invalid email address" even though it is valid. I have searched Opencart and can see that other people have had similar issues. Can anybody tell me how to fix this? Please note I am not a developer. I can log into the backend of the website but from there I have no idea where to click and what to do to start fixing this. We paid an enormous amount of money to have this website developed and now the developers are pulling up their shoulders. Please if anybody is willing to help.

I will help you but you need to pay some money .Because there is problem in your login script and need to debug it.