I have created a custom field in CRM, for the use of storing an emailaddress for invoice purposes. The account manager enters this field in CRM (he doesn't have access to ZoHo Invoice). I want to synchronize this custom field so that the invoice emailaddress is imported in ZoHo Invoice and it will be used for sending the invoice to the customer via email automatically (default invoice email address). 

Currently, the emailaddress of the primary contact of the customer is being used, but this is rarely the correct invoice emailaddress. 

Please advise how I can synchronize this custom field into ZoHo Invoice. Because in the synchronization settings, I cannot change the "email" field.