Invalid Attendee Email

Hey All
Can anyone possibly help me with this. Been seeing it for a year now and would just be great to resolve. Already checked the logs and cant find anything there. Just this message

11:36:46 PM MailExceptions.OperationException: Uploading item(s) to folder ‘johnobennett@gmail.com/default/’ failed with an error: Invalid attendee email. —> The request ‘Google.Apis.Requests.Request(insert @ https://www.googleapis.com/calendar/v…)’ has failed. The service calendar has thrown an exception: Google.GoogleApiRequestException: Google.Apis.Requests.RequestError
11:36:46 PM Invalid attendee email. [400]
11:36:46 PM Errors [
11:36:46 PM Message[Invalid attendee email.] Location[-] Reason[invalid] Domain[global]
11:36:46 PM ]
11:36:46 PM —> System.Net.WebException: The remote server returned an error: (400) Bad Request.
11:36:46 PM at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
11:36:46 PM at Google.Apis.Requests.Request.InternalEndExecuteRequest(IAsyncResult asyncResult)
11:36:46 PM — End of inner exception stack trace —
11:36:46 PM at Google.Apis.Requests.ServiceRequest1.EndFetchInternal(IAsyncResult asyncResult) 11:36:46 PM at Google.Apis.Requests.ServiceRequest1.EndFetch(IAsyncResult asyncResult)
11:36:46 PM at MailClient.Protocols.Gdata.FetchHelper.FetchWithCancellation[T](ServiceRequest1 request, CancellationToken cancellationToken) 11:36:46 PM at MailClient.Protocols.Gdata.CalendarItemSynchronizer.Upload(IItemUploadContext1 uploadContext, Folder folder, IEnumerable1 items, CancellationToken cancellationToken) 11:36:46 PM at MailClient.Protocols.Common.ItemSynchronizer2.<>c__DisplayClass22.b__1e(WorkerStatus status, CancellationToken cancellationToken)
11:36:46 PM at MailClient.Protocols.Gdata.GenericGapiCommand.Execute(WorkerStatus status)
11:36:46 PM — End of inner exception stack trace —
11:36:46 PM at MailClient.Protocols.Gdata.GenericGapiCommand.Execute(WorkerStatus status)

Hi,
can you make a screenshot of the error you’re receiving?
Thank you,
Paul.