Yeah okay, but I should have added that during the (long) time when the emails were being sent correctly, the backups never failed. So I had a high confidence everything was ok.
I don't remember the exact specifics but it had to do with filename encodings. One file had special characters in its name, rsync on Windows (DeltaCopy) was reporting it existing, but it could not be transmitted and so the remote server kept asking for it, or something like that, and after some back and forth the whole process failed.