$ telnet localhost pop3
Trying ::1...
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused


$ netstat -l

tcp 0 0 *:www : LISTEN
tcp 0 0 localhost.localdoma:ipp : LISTEN
tcp 0 0 *:smtp : LISTEN
tcp 0 0 localhost.localdo:mysql : LISTEN

and nothing about dovecot in mail.log or mail.err

when I run this service dovecot start I got

start: Rejected send message, 1 matched rules; type="method_call", sender=":1.553" (uid=1000 pid=26250 comm="start) interface="com.ubuntu.Upstart0_6.Job" member="Start" error name="(unset)" requested_reply=0 destination="com.ubuntu.Upstart" (uid=0 pid=1 comm="/sbin/init"))

in dovecot.conf

protocols = imap imaps pop3 pop3s
disable_plaintext_auth = no
log_timestamp = "%Y-%m-%d %H:%M:%S "
mail_location = maildir:/var/spool/mail/%d/%n
mail_access_groups = mail
first_valid_uid = 106
first_valid_gid = 106
protocol imap {

}

protocol pop3 {
listen=*:110
pop3_uidl_format = %08Xu%08Xv
}
protocol lda {
  postmaster_address = [email protected]
  mail_plugins = quota
  log_path = /var/log/dovecot-deliver.log
  info_log_path = /var/log/dovecot-deliver.log
}

auth default {
mechanisms = digest-md5 plain
passdb sql {
args = /etc/dovecot/dovecot-mysql.conf
}
userdb sql {
args = /etc/dovecot/dovecot-mysql.conf
}
user = root
}