[Dovecot] too many files opened

[Joshua, C.S. Chen]

Hi folks,
My institute's mail server was upgraded to RHEL5 with dovecot 1.0-2.1rc5
recently. And it uses PAM authentication which points to LDAP server.
Now from time to time it gets Error like

dovecot: Aug 29 09:23:59 Error: auth(default):
pam(joshua,192.168.177.52): pipe() failed: Too many open files


and users can not login. My config is like

dovecot -n

# /etc/dovecot.conf
base_dir: /var/run/dovecot/
log_path: /var/log/dovecot
info_log_path: /var/log/dovecot
listen: *
login_dir: /var/run/dovecot/login
login_executable(default): /usr/libexec/dovecot/imap-login
login_executable(imap): /usr/libexec/dovecot/imap-login
login_executable(pop3): /usr/libexec/dovecot/pop3-login
login_greeting: ASIAA Dovecot ready.
login_processes_count: 4
mail_location:
mbox:/asiaa/home/%u/:/asiaa/home/%u/mail/:INBOX=/var/mail/%u:INDEX=/asiaa/home/%u/mail/dovecot_indexes
mail_debug: yes
mail_full_filesystem_access: yes
mail_drop_priv_before_exec: yes
mail_executable(default): /etc/dovecot_postlogin_beforeimap.sh
mail_executable(imap): /etc/dovecot_postlogin_beforeimap.sh
mail_executable(pop3): /etc/dovecot_postlogin_beforepop3.sh
mail_process_size: 512
mail_plugin_dir(default): /usr/lib/dovecot/imap
mail_plugin_dir(imap): /usr/lib/dovecot/imap
mail_plugin_dir(pop3): /usr/lib/dovecot/pop3
pop3_uidl_format(default): %08Xu%08Xv
pop3_uidl_format(imap): %08Xu%08Xv
pop3_uidl_format(pop3): %08Xv%08Xu
auth default:
passdb:
driver: pam
userdb:
driver: passwd




Any suggestion or help is appreciated
Cheers
Joshua