我安装了 Ubuntu Server 18.04，并根据 Ubuntu 文档网站上的 Ubuntu Server 指南安装了 Apache、MySQL、PhpMyAdmin、Postfix、Dovecot、Mailman 等，但尝试通过 Squirrelmail 发送电子邮件时出现此错误：
Message not sent. Server replied:
Requested action aborted: error in processing
451 4.3.0 <root@localhost>: Temporary lookup failure
还：
dns@mycroft:/etc/postfix$ telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 mycroft ESMTP Postfix (Ubuntu)
helo mycroft
250 mycroft
mail from: dns@localhost
250 2.1.0 Ok
rcpt to: root@localhost
451 4.3.0 <root@localhost>: Temporary lookup failure
这是我的 /etc/postfix/main.cf
# See http://www.postfix.org/COMPATIBILITY_README.html -- default to 2 on
# fresh installs.
compatibility_level = 2
# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = mycroft
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
mydestination = localhost, localhost.localdomain
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter =
inet_interfaces = loopback-only
default_transport = error
relay_transport = error
inet_protocols = ipv4
home_mailbox = Maildir/
relay_domains = localhost
transport_maps = hash:/etc/postfix/transport
mailman_destination_recipient_limit = 1
myorigin = localhost
我只想在主机本地发送邮件，因此此时没有任何出站邮件，并且我无法在服务器上从一个用户发送到另一个用户。
任何帮助都将不胜感激。谢谢！
· 利维