[dns-operations] MX CNAMEs

[风河]

I was thinking MX should not be a CNAME record.
But these two are broken the rules, and they seem work just fine?

2008.sina.com.        52    IN    MX    10 mx.2008.sina.com.

mx.2008.sina.com.    8    IN    CNAME    freemx1.sinamail.sina.com.cn.
freemx1.sinamail.sina.com.cn. 8    IN    A    202.108.3.242

And,

mail.china.com.        391    IN    CNAME    mail.china.com.cachecn.com.
mail.china.com.cachecn.com. 191    IN    MX    10 
mx-mail-china-com.icoremail.net.

Thanks.