Just took a look and found 366 files in the \SmarterMail\Service\App_Data\Attachments folder. The oldest one is from three days ago, so it seems like a self-cleaning cache.

I dont have that folder at all...

Its at;

C:\Program Files (x86)\SmarterTools\SmarterMail\Service\App_Data\Attachments

I believe the path differences noted by other commenters is likely due to install specifics, and this is the normal attachments folder within the Service directory. That said, I'm betting your antivirus isn't configure to exclude that directory and is locking those files on upload/sync. I recommend excluding those folders from Windows Defender and any third-party AV solutions you run alongside it. If you'd like to incorporate AV scanning of attachments and emails this should be done within SmarterMail in Settings>Antivirus once those exclusions have been implemented. We have a KB article on this you can reference as well and you can find that here: