SMTP connection from [xx.xx.xx.xx]:33143 (TCP/IP connection count = 1)

2014-11-27 18:58:50 dovecot_login authenticator failed for xx.xx.xx.xx ([x.xx.xx]) [xx.xx.xx.xx]:33143: 435 Unable to authenticate at present: authentication socket connection error

# 2.2.13: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.18-274.18.1.el5 x86_64 CentOS release 5.11 (Final)
auth_cache_size = 1 k
auth_mechanisms = plain login
auth_username_chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!#$-=?^_{}~./@+%"
disable_plaintext_auth = no
namespace {
  inbox = yes
  location =
  prefix = INBOX.
  type = private
}
passdb {
  args = /usr/local/cpanel/bin/dovecot-wrap
  driver = checkpassword
}
plugin {
  acl = vfile
  quota = maildir
  quota_rule = INBOX.Trash:ignore
}
protocols = imap pop3
service auth {
  unix_listener auth-client {
    mode = 0666
  }
}
service imap-login {
  client_limit = 500
  inet_listener imap {
    address = *
  }
  inet_listener imaps {
    address = *
  }
  process_limit = 50
  process_min_avail = 2
  service_count = 0
  vsz_limit = 64 M
}
service imap {
  process_limit = 512
  vsz_limit = 256 M
}
service managesieve-login {
  client_limit = 500
  process_limit = 50
  process_min_avail = 2
  service_count = 0
  vsz_limit = 64 M
}
service managesieve {
  process_limit = 512
  vsz_limit = 256 M
}
service pop3-login {
  client_limit = 500
  inet_listener pop3 {
    address = *
  }
  inet_listener pop3s {
    address = *
  }
  process_limit = 50
  process_min_avail = 2
  service_count = 0
  vsz_limit = 64 M
}
service pop3 {
  process_limit = 512
  vsz_limit = 256 M
}
ssl_cert = </etc/dovecot/ssl/dovecot.crt
ssl_cipher_list = ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:-LOW:-SSLv2:-EXP
ssl_key = </etc/dovecot/ssl/dovecot.key
ssl_protocols = !SSLv2 !SSLv3
userdb {
  driver = prefetch
}
userdb {
  args = /usr/local/cpanel/bin/dovecot-wrap
  driver = checkpassword
}
protocol imap {
  imap_capability = +NAMESPACE
  imap_idle_notify_interval = 24 mins
  imap_logout_format = in=%i, out=%o, bytes=%i/%o
  mail_max_userip_connections = 20
  mail_plugins = acl quota imap_quota
}
protocol pop3 {
  mail_max_userip_connections = 3
  mail_plugins = quota
  pop3_logout_format = top=%t/%p, retr=%r/%b, del=%d/%m, size=%s, bytes=%i/%o
  pop3_uidl_format = UID%u-%v
}
protocol lda {
  postmaster_address = postmaster@example.com
}