我已经在 CentOS 7 上使用 Postfix、Dovecot 和 MariaDB 设置了电子邮件服务器，但是邮件日志中出现错误，这是什么原因以及如何解决？
错误信息：
May 14 01:51:27 localhost postfix/master[1798]: warning: /usr/libexec/postfix/proxymap: bad command startup -- throttling
May 14 01:51:27 localhost postfix/submission/smtpd[26968]: warning: private/proxymap socket: service dict_proxy_open: Success
May 14 01:51:27 localhost postfix/master[1798]: warning: process /usr/libexec/postfix/proxymap pid 13088 exit status 1
May 14 01:52:27 localhost postfix/proxymap[13126]: fatal: /etc/postfix/mysql-virtual_mailboxes.cf: bad string length 0 < 1: table =
May 14 01:52:27 localhost postfix/proxymap[13127]: fatal: /etc/postfix/mysql-virtual_mailboxes.cf: bad string length 0 < 1: table =