要给namecheap的域名设置MX记录，还略麻烦，直接设置不生效，于是乎看看文档，发现：
NOTE:Before setting up your mail service, be sure that there is no CNAME record created for abare domain, (e.g.,yourdomain.tld) in theHost recordssection, or email will not work correctly. CNAME has the highest priority and prevails over all the other records for the host name, including MX Records which are responsible for mail delivery.
意思是说，设置邮件MX记录之前，不能再host 记录里面有顶级域名(一级域名)的CNAME记录，否则不会MX生效，因为CNAME优先级别最高。
也就是说，先把CNAME删除，在设置就OK了。设置成功后，在把CNAME加上即可。
