Email not being sent from Website admin

Our website is using OSCommerce admin site. For this month, we have an issue where emails are not being sent to our customer users. When a new user signs up in the website or requests for a new password, no email response or confirmation was being received by the user. When a new order is being made in the website, confirmation of the order is not being received either. We've checked this with our hosting site support and this is their response: Issue appears to be indeed due to the fact the the OsCommerce application sends messages that contain two TO RECIPIENT fields in its header. I have checked your application's admin area and the provided options in there trying to find a way to change this setting, however I could not. To resolve the issue I would advise you to setup messages sent via the application to have only one TO field Please send us your price quote to resolve this issue. If need further details, please let us know.