How should I setup separate MX records for a subdomain?

Solution 1:

You should never point your MX to a IP address to be RFC compliant. Make an A record for the IP address instead and point the MX record to it.

Then the zone should look like this,

    @               IN      MX      1       ASPMX.L.GOOGLE.COM.
    @               IN      MX      5       ALT1.ASPMX.L.GOOGLE.COM.
    @               IN      MX      5       ALT2.ASPMX.L.GOOGLE.COM.
    @               IN      MX      10      ASPMX2.GOOGLEMAIL.COM.
    @               IN      MX      10      ASPMX3.GOOGLEMAIL.COM.
    @               IN      MX      10      ASPMX4.GOOGLEMAIL.COM.
    @               IN      MX      10      ASPMX5.GOOGLEMAIL.COM.
    @               IN      A       10.24.233.214
    mailer          IN      A       10.24.233.214
    mailer          IN      MX      10      mailer.cranketywidgets.com.

Solution 2:

Do you intend for people to send mail to addresses at @mailer.crankywidgets.com? If not, then you don't need MX records for it. MX plays no role in mail sent from that domain.

Setting up a separate mail server to send mail from addresses @mailer.crankywidgets.com is the right path, and you should set up a proper A record, PTR if the host is dedicated, and SPF records saying that that given IP address is allowed to send mail for the domain.

If all the mail sent out is intended to be notification style, you can set the reply-to address to something like "[email protected]" and then blackhole that on Google Apps.

The only thing I can see a MX record buying you here is a slightly different SPF record, as you can say "the MX (email receiver) hosts for this domain are also allowed to send mail from this domain". But SPF is versatile enough to say "this arbitrarily named host is allowed to send mail from this domain" too.

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Solution 3:

As mail is a minefield.

PTR <-> A <-> MX

records should match for perfect reverse lookups on some spam mail filters.

https://en.wikipedia.org/wiki/Reverse_DNS_lookup