改写：CNAME记录就是一个替换，所以域名一旦设置CNAME记录以后，就不能再设置其他记录了（比如A记录和MX记录），这是为了防止产生冲突。举例来说，foo.com指向bar.com，而两个域名各有自己的MX记录，如果两者不一致，就会产生问

A CNAME record serves as a substitution, so once a domain has been set with a CNAME record, no other records can be set (such as A records and MX records) to prevent conflicts. For example, if foo.com points to bar.com, and both domains have their own MX records that are inconsistent, problems may arise. Since top-level domains usually require MX records to be set, users are generally not allowed to set CNAME records for top-level domains.