ubuntu 12.04 dovecot ssl 已启用，但 ssl_cert 未设置

Question

其详细信息如下：

doveconf -n
# 2.0.19: /etc/dovecot/dovecot.conf
# OS: Linux 2.6.32-042stab120.20 x86_64 Ubuntu 12.04.5 LTS ext4
auth_socket_path = /var/run/dovecot/auth-userdb
first_valid_gid = 6
first_valid_uid = 26
hostname = %d
lda_mailbox_autocreate = yes
lda_mailbox_autosubscribe = yes
listen = *
mail_gid = mail
mail_location = maildir:/var/mail/%d/%n
mail_plugin_dir = /usr/local/lib/dovecot
mail_uid = mailnull
passdb {
  args = /usr/local/etc/dovecot/dovecot.sql
  driver = sql
}
postmaster_address = postmaster@%d
protocols = imap
sendmail_path = /usr/local/sbin/exim
service auth {
  unix_listener auth-userdb {
    group = $mail_gid
    mode = 0600
    user = $mail_uid
  }
}
service imap-login {
  inet_listener imap {
    port = 143
  }
}
service pop3-login {
  inet_listener pop3 {
    port = 110
  }
}
userdb {
  args = /usr/local/etc/dovecot/dovecot.sql
  driver = sql
}
doveconf: Error: ssl enabled, but ssl_cert not set
protocol pop3 {
  pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
}
protocol imap {
  imap_client_workarounds = delay-newmail tb-extra-mailbox-sep tb-lsub-flags
  mail_plugins = quota imap_quota antispam
}
doveconf: Fatal: Error in configuration file /etc/dovecot/dovecot.conf: ssl enabled, but ssl_cert not set