[Dovecot] dovecot-lmtp does not work

[He Shiming]

Dear Community,

I've got a weird problem regarding lmtp setup with dovecot 2.0.19 on ubuntu
12.04.2 LTS. My reference of the configuration is at
https://library.linode.com/email/postfix/postfix2.9.6-dovecot2.0.19-mysql ,
and my configuration is identical to it with the exception of opening the
110 pop3 port, and enabling plain text auth. Everything from imap, smtp,
pop3 (including auth) works except for lmtp.

I saw thousands of the following messages in /var/log/mail.log:

May 31 09:05:24 postfix/lmtp[3664]: 2FD80321F4: to=<myaddress at me.com>,
relay=none, delay=0.41, delays=0.41/0/0/0, dsn=4.4.1, status=deferred
(connect to me.com[private/dovecot-lmtp]: No such file or directory)

And it happens when an incoming email is received.

Here's the output of configuration:

root at prosp:/etc/dovecot/conf.d# dovecot -n
# 2.0.19: /etc/dovecot/dovecot.conf
# OS: Linux 3.8.4-linode50 i686 Ubuntu 12.04.2 LTS ext3
auth_mechanisms = plain login
disable_plaintext_auth = no
mail_location = maildir:/var/mail/vhosts/%d/%n
mail_privileged_group = mail
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
passdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
plugin {
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve
}
protocols = imap pop3 sieve
service auth-worker {
  user = vmail
}
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0666
    user = postfix
  }
  unix_listener /var/spool/postfix/private/dovecot-auth {
    group = postfix
    mode = 0660
    user = postfix
  }
  unix_listener auth-userdb {
    mode = 0600
    user = vmail
  }
  user = dovecot
}
service imap-login {
  inet_listener imap {
    port = 0
  }
}
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}
service pop3-login {
  inet_listener pop3 {
    port = 110
  }
}
ssl_cert = </etc/ssl/certs/dovecot.pem
ssl_cipher_list =
ALL:!LOW:!SSLv2:ALL:!aNULL:!ADH:!eNULL:!EXP:RC4+RSA:+HIGH:+MEDIUM
ssl_key = </etc/ssl/private/dovecot.pem
userdb {
  args = uid=vmail gid=vmail home=/var/mail/vhosts/%d/%n
  driver = static
}
protocol imap {
  imap_client_workarounds = delay-newmail
  mail_max_userip_connections = 10
}
protocol pop3 {
  mail_max_userip_connections = 10
  pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
}
protocol lda {
  deliver_log_format = msgid=%m: %$
  mail_plugins = sieve
  postmaster_address = postmaster
  quota_full_tempfail = yes
  rejection_reason = Your message to <%t> was automatically rejected:%n%r
}

This problem has been discussed here:
http://serverfault.com/questions/512219/dovecot-lmtp-does-not-exist . I've
then tried switching from unix socket to inet listener, and still got the
same result. I cannot get lmtp service to start.

I'm also seeing no errors in the log when dovecot is restarted (see the
above link for the log).

I'm wondering how do I go about troubleshooting this problem? What might be
the possible cause? Is it possible for dovecot to output verbose logs
regarding service startups?

Thank you.

-- 
Best regards,
He Shiming
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