When synching Invoices to Xero - you may encounter the following error message 'Email address must be valid', this occurs when the system tries to create a new contact but the email address saved in KennelBooker is invalid,
This may be something simple such as an additional dot being placed into the domain accidentally
eg janedoe@googlemail..com
If you click into the customers account in KennelBooker and fix the mistake/error the record should then sync.