Why the sync error occurs
This error occurs when a customer in QuickBooks Online is trying to sync to Bill.com, but there are multiple email addresses in the email field.
How to fix the sync error
Delete all but one email address for the customer in QuickBooks Online, and sync.
To prevent this error, enter only one email address for each customer when entering the customer details in QuickBooks Online.
You can only have one email address as the main point of contact for a customer, but you can add additional customer contacts after it syncs to Bill.com.