On each row of a subform I want to check whether the customer's email address already exists in our database. 

FORM = Customers (this contains contact details etc including email)

MAIN FORM = Clinics 

SUBFORM = Sign-up

Something like: 

For each row in Sign-up

if(Customers[Email_customers == input.Email_sign_up].count() == 0)

{

alert "This email address does not exist in our records. Please register";

} 

I'm not a seasoned coder so I'm getting a bit confused with rows, subrows, recs etc so I know I'm some way off with the code! Can anyone help please?