[dns-operations] Can MX be working with CNAME?

[Florian Weimer]

* Paul Vixie:

> while it's true that it is worse to have a cname used to link an MX to
> its target, it is not true that pointing a CNAME at an MX will nec'ily
> end well. in the above example, Sendmail in its default configuration
> will rewrite on the next hop the From: header so that it shows
> @plus-china.l.google.com.

That's a sendmail bug.  But RFC 1123 actually requires that you
rewrite the MAIL FROM and RCPT TO arguments in the SMTP envelope (see
section 5.2.2).

> i think this is not the behaviour that google's trying to achieve
> here. and Sendmail may or may not be wrong to rewrite headers, but
> it's the default config, just the same.

Wrong for headers, right for the envelope, historically speaking.
According to RFC 5321, rewriting is no longer correct.