ContractorTools writes customer email address to QuickBooks, but with the following caveats:
While ContractorTools allows you to enter any number of email addresses, and each email address can have a different "label" (to indicate what the email address is for: home, work, etc.), only one email address will be selected (at random) to be written to QuickBooks.
The email address that gets written to QuickBooks must have a label that is either "Home", "Work", or "Other". If you set the label of the email address to anything else (such as a custom label), it won't be eligible to be randomly selected to get written to QuickBooks.