Comment 1 for bug 265748

arch only handles mbox files in "Unix mbox format". Such
files delineate messages with a single line containing "From
" -- that's "F R O M space" at the front of the line.
There
can be stricter criteria on what follows that header, but
that's the gist of it. Maybe your individual files don't
have the right separator.

You can try running bin/cleanarch on the file to see if it's
close. Use --dry-run to just examine the file.