concerning dovecot settings for high volume server

[Rajesh M]

hi

centos 6 64 bit

hex core processor with hyperthreading ie display shows 12 cores
16 gb ram
600 gb 15000 rpm drive

we are having around 4000 users on a server


i wish to allow 1500 pop3 and 1500 imap connections simultaneously.

need help regarding the settings to handle the above

imap-login, pop3-login
imap pop3 service settings

i recently i got an error 
imap-login: Error: read(imap) failed: Remote closed connection (process_limit reached?)


my current dovecot config file

# 2.2.7: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-431.23.3.el6.x86_64 x86_64 CentOS release 6.5 (Final)
auth_cache_negative_ttl = 0
auth_cache_ttl = 0
auth_mechanisms = plain login digest-md5 cram-md5
default_login_user = vpopmail
disable_plaintext_auth = no
first_valid_gid = 89
first_valid_uid = 89
log_path = /var/log/dovecot.log
login_greeting = ready.
mail_max_userip_connections = 50
mail_plugins = " quota"
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character vacation subaddress comparator-i;ascii-numeric relational regex imap4flags copy include variables body enotify environment mailbox date ihave
namespace {
  inbox = yes
  location =
  prefix =
  separator = .
  type = private
}
passdb {
  args = cache_key=%u webmail=127.0.0.1
  driver = vpopmail
}
plugin {
  quota = maildir:ignore=Trash
  quota_rule = ?:storage=0
}
protocols = imap pop3
service imap-login {
  client_limit = 256
  process_limit = 400
  process_min_avail = 4
  service_count = 0
  vsz_limit = 512 M
}
service pop3-login {
  client_limit = 1000
  process_limit = 400
  process_min_avail = 12
  service_count = 0
  vsz_limit = 512 M
}
ssl_cert = </var/qmail/control/servercert.pem
ssl_dh_parameters_length = 2048
ssl_key = </var/qmail/control/servercert.pem
userdb {
  args = cache_key=%u quota_template=quota_rule=*:backend=%q
  driver = vpopmail
}
protocol imap {
  imap_client_workarounds = delay-newmail
  mail_plugins = " quota imap_quota"
}
protocol pop3 {
  pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
  pop3_fast_size_lookups = yes
  pop3_lock_session = no
  pop3_no_flag_updates = yes
}


thanks very much,

rajesh