As long as you are using a string for the Account property, the Get Outlook Email Messages action should not throw any errors. Maybe this sounds a bit too basic but are you sure you put the email address in quotes ?
System.ArgumentException: The specified folder does not exist
at UiPath.Mail.Activities.GetMailActivity.EndExecute(AsyncCodeActivityContext context, IAsyncResult result)
at System.Activities.AsyncCodeActivity.System.Activities.IAsyncCodeActivity.FinishExecution(AsyncCodeActivityContext context, IAsyncResult result)
at System.Activities.AsyncCodeActivity.CompleteAsyncCodeActivityData.CompleteAsyncCodeActivityWorkItem.Execute(ActivityExecutor executor, BookmarkManager bookmarkManager)