This is an error in which will prompt when the Preferred Delivery Method for the Customer in your QuickBooks Company File is set to 'E-mail' and there is not an e-mail listed under the Customer's information or the Customer's Jobsites.
Simply change the Preferred Delivery Method away from 'E-mail', to either 'Mail', or 'None', as shown above, or, if you wish to use e-mail to deliver invoices, add an email to the client's file (and keep the Preferred Delivery Method as 'E-mail').
Make sure to look at the Jobsites under the Customer as well, if you are still getting the error after checking at the Customer level. It is possible that the Customer's Jobsite has a Preferred Delivery Method set to 'E-mail' as well, hence why the error is still showing when you try to import invoices over to QuickBooks, even though you changed the method on the Customer level.
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