Cause
You get this error when saving email to disk as single individual files, and the windows allowed filename length is exceeded.
Windows filenames are limited to 255 characters for the filename *plus* the path.
Resolution
- Select a top folder with a short name for the destination directory, like d:\mail
- Use a filename format that is limited in its length like
- [submitdate][from]-id-subject.* (96 characters)
- uniqueIdid-subject.* (64 characters)
- uniqueIdid.* (31 characters)