Adriel via Postfix-users:
> Hello
>
> say i have a subdomain sub.xyz.com.
>
> if I make a CNAME as,
>
> sub.xyz.com CNAME to xyz.net
>
> and, xyz.net has its own MX and SPF records.
>
> my question is, for this DNS setup, will sub.xyz.com uses MX and SPF of
> xyz.net for its mail hosting?
>
> for example, when external users write to u...@sub.xyz.com, the messages
> will route to xyz.net's MX server. And, when u...@sub.xyz.com deliver
> message out, the peer MTA will use xyz.net's SPF for validation. Am I
> right?
That depends on what the MX record points to. If you're trying to
do this, then that violates RFC 5321:
example.com. IN A 10.0.0.1
example.com. IN MX 10 mail.example.com.
mail.example.com. IN CNAME example.com
(an MX record must not point to a CNAME record).
Quote from RFC 5321: "If an MX MX RR is looked up and the associated
data field obtained:
- The data field of that response MUST contain a domain name.
- That domain name, when queried, MUST return at least one address
record (e.g., A or AAAA RR) that gives the IP address of the SMTP
server to which the message should be directed." End of quote.
The Postfix DNS client does not care if some domain name comes from
the data field of an MX record. There is, however, no guarantee
that other MTAs will also allow CNAME to be used in this way.
Wietse
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