unable to opendir mess

[vincent]

Hi my qmail have been running well for a long time, but recently,when i check the maillog,error occurs: Jul 27 12:21:57 webserver qmail: 996265317.412854 alert: unable to opendir mess/5, sleeping... Jul 27 12:22:07 webserver qmail: 996265327.422859 alert: unable to opendir mess/5, sleeping... the qmail can not deliver mail out,all mails are in /var/qmail/queue/mess.what's wrong with that?