我正在检查日志，在邮件日志中发现了以下几行
postfix/smtpd[12135]: warning: hostname mta9.imxonlines.co.za does not resolve to address 91.212.150.89: Name or service not known
connect from unknown[91.212.150.89]
disconnect from unknown[91.212.150.89]
我经常看到这些行。这是某种 smtp 攻击吗？如果是，那么我该如何防止这些连接和断开连接？