So we need more input what you are doing. Is there a way you can reproduce the case when the "error" occurs? You wrote with a clean new $user mbox file there is no problem. So it will be very good to know what you did just before the error in maillog occurs. You may to precisely describe your mail setup (including procmail filter script) and which actions you run.
Check the permissions and ownership (user/group) not only of the files, but also of the parent directory. That one bit me a few times.
-- Rodolfo J. Paiz rpaiz@xxxxxxxxxxxxxx http://www.simpaticus.com
