mailmanのインストールがなかなか上手くいかないので、virtual_mailboxをやめてunixホームディレクトリを使用する設定に変更した。変更後の修正点をまとめると以下の通り。
/etc/postfix/main.cfはmain.cf.protoから以下の部分を変更。

mail_owner = postfix
myhostname = xxx.moon-forest.com
mydomain = moon-forest.com
myorigin = /etc/mailname
inet_interfaces = all
mydestination = $myhostname, $mydomain, localhost.$mydomain, localhost
local_recipient_maps = unix:passwd.byname $alias_maps
alias_maps = hash:/etc/aliases
home_mailbox = Maildir/
sendmail_path = /usr/sbin/postfix
newaliases_path = /usr/bin/newaliases
mailq_path = /usr/bin/mailq
setgid_group = postdrop
#html_directory =
#manpage_directory =
#sample_directory =
#readme_directory =
message_size_limit = 33554432
mailbox_size_limit = 1073741824
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
smtpd_sasl_local_domain = $myhostname
smtpd_recipient_restrictions = 
      permit_mynetworks,
      permit_auth_destination,
      permit_sasl_authenticated,
      reject

/etc/postfix/master.cfはmaster.cf.protoから以下の部分を変更。

listen = *, ::

/etc/dovecot/conf.d/10-auth.confは以下の部分を変更。

disable_plaintext_auth = no
auth_mechanisms = cram-md5 plain login
#!include auth-system.conf.ext
!include auth-passwdfile.conf.ext

/etc/dovecot/conf.d/10-mail.confは以下の部分を変更。

mail_location = maildir:~/Maildir

/etc/dovecot/conf.d/10-master.conf

service imap-login { 
  inet_listener imap {
  }
  inet_listener imaps {
  }
}

service pop3-login {
  inet_listener pop3 {
  }
  inet_listener pop3s {
  }
}

service lmtp {
  unix_listener lmtp {
  }
}

service imap {
}

service pop3 {
}

service auth {
  unix_listener auth-userdb {
  }
  # Postfix smtp-auth
  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
    user = postfix
    group = postfix
  }
}

service auth-worker {
}

service dict {
  unix_listener dict {
  }
}
/etc/dovecot/conf.d/auth-passwdfile.conf.ext
passdb { 
  driver = passwd-file
  args = scheme=CRAM-MD5 username_format=%u /etc/dovecot/users
}

userdb {
  driver = passwd-file
  args = username_format=%u /etc/dovecot/users
}

これ以外の設定ファイルはデフォルトに戻した。