"You have crossed the maximum number of contact persons that can be added."

"You have crossed the maximum number of contact persons that can be added."

One of my clients has changed her email address but I need to add her new email address to the "Email to" at the bottom of the invoice page. But when I tried to add her new details to the "+ Add New" I get the following message:

"You have crossed the maximum number of contact persons that can be added."

I went in to the customer section and saw I had 4 "CONTACT PERSONS", so I deleted one and tried again to add her new details in again... still got the same error message. 

How can I fix this issue as she, along with the accounts payable department need to both get the invoice?