Seeing failed in the top line of an HDR file means that there was an issue with the session in SMTP. Either it took too long, was prematurely disconnected, or was rejected by the server for some reason. We have a process that runs through and cleans these up. These not clearing out may have something to do with the spool issues you were having. It would be interesting to know if it is still stacking up or if our cleanup process, which is part of the spool processing loop, is acting up.