How do I set my MX records such that mail to my domain goes one place, and mail to all subdomains goes elsewhere?

Question

I have a domain name example.com, for which I'd like mail to be routed to different servers depending on the circumstances.

Mail sent to an @example.com address should go to the server mail.server1.com, and mail to any subdomain @*.example.com should to to mail.server2.com

Is there a way to configure MX records to do this (or any other way to do this for that matter)? Can I use wildcard DNS entries for MX records?

Answer 1

I discovered the answer to this almost immediately after posting :)

Set the MX records for example.com to mail.server1.com as expected.

Then to set the wildcard, you can create a wildcard CNAME *.example.com which points to some generic subdomain sub.example.com, then create an MX record for sub.example.com which points to mail.server2.com.

Answer 2

A wildcard MX record is perfectly valid, there's no need for your extra CNAME.

You could simply have...

example.com    IN MX 10 mail.server1.com
*.example.com  IN MX 10 mail.server2.com

Your CNAME just causes an extra DNS lookup

Answer 3

It all depends on the DNS server you are using.

In Bind from this example:

; zone fragment for 'zone name' example.com
; name servers in the same zone
$TTL 2d ; zone default TT = 2 days
$ORIGIN example.com.
@              IN      SOA   ns1.example.com. hostmaster.example.com. (
               2003080800 ; serial number
               2h         ; refresh =  2 hours 
               15M        ; update retry = 15 minutes
               3W12h      ; expiry = 3 weeks + 12 hours
               2h20M      ; minimum = 2 hours + 20 minutes
               )
; main domain name servers
              IN      NS     ns1.example.com.
              IN      NS     ns2.example.com.
; mail servers for main domain
              IN      MX 10  mail.example.com.
; A records for name servers above 
ns1           IN      A      192.168.0.3
ns2           IN      A      192.168.0.4
; A record for mail servers above 
mail          IN      A      192.168.0.5
; other domain level hosts and services
bill          IN      A      192.168.0.6
....
; sub-domain definitions
$ORIGIN us.example.com.
              IN      MX 10  mail
; record above uses blank substituition 
; and could have been written as 
; us.example.com.   IN  MX 10 mail.us.example.com.
; OR (using @ substitution)
; @ IN MX 10 mail
; A record for subdomain mail server
mail          IN      A      10.10.0.28
; the record above could have been written as 
; mail.us.example.com. A 10.10.0.28 if it's less confusing
ftp           IN      A      10.10.0.29 
; the record above could have been written as 
; ftp.us.example.com. A 10.10.0.29 if it's less confusing
....
; other subdomain definitions as required 

Additional sub-domains could be defined in the same file using the same strategy. For administrative convenience you could use $INCLUDE directives e.g.

; snippet from file above showing use of $INCLUDE
....
; other domain level hosts and services
bill          IN      A      192.168.0.5
....
; sub-domain definitions
$INCLUDE us-subdomain.sub
; other subdomain definitions as required