contact syncing error

For more than a year now I’ve had the below error, which gives me grief when trying to setup groups and sync contacts.  Any ideas on correcting this error?

9:32:50 AM   MailClient.Storage.Application.OperationException: Uploading item(s) to folder ‘’ failed due to the following error: Execution of request failed: ([Line 9, Column 31, element gd:email] g:email/@address is required)