We had this issue with Declude as well (which we blamed on them), but using the custom headers service, with no changes, simply compile, install the service, start. SM leaves behind at random .hdr files. I let it run for an hour or so and within that time, SM left behind over 1500 .hdr files. It does not clean them up, not sure why it does not remove them.
I am wondering if it is something to do with the service moving the files to the drop directory at the same time the SM service is trying to pick up the files. Which either results in SM not seeing the .hdr file or trying to read it while the service that moved the file has not yet released the file.
Which makes me wonder if 1. Does SM even read the HDR file? If not, what does that mean for that email? 2. If SM fails to delete, maybe retry?
I can make the service delete the stale files, but why... I am assuming it is some timing issue with SM reading the directory and the custom headers service moving the files into the directory, since SM does not leave orphaned .hdr files in the regular spool directories.