If this is an '''imap''' mail account, then the 'Trash' folder may be a folder on the server which you have subscribed to see and it is being emptied by the server. Many servers will allow deleted emails to exist in Trash for a short period just in case one was deleted in error, but after a given period of time they get auto deleted because they are in trash and you would only put it in the trash if you did not want it. It is a way of the server reclaiming some space by not keeping unwanted mail for long periods.