I've seen this happen when the receipt of the email gets interrupted or terminated prematurely to where you have the .hdr file for the message but no corresponding .eml file for the message (I believe this can also happen if your Anti-Virus has to delete the .eml when it can't remove the infection). Smartermail cannot attempt to deliver the message if no message exists, resulting in the item sitting in your queue. In such cases of orphaned .hdr files (or in the inverse of orphaned .eml files) you would just delete the items from your \Smartermail\Spool\SubSpool# directory.