creating MX records for subdomain

[Bell, William IT]

Hi,
We need to create a subdomain within our parent domain.  This subdomain is
only necessary to provide a new product with an avenue to do a DNS lookup on
an MX record for this new subdomain so they can deliver mail to a specific
mail server (this is how the new software determines which mail server email
should be delivered).  So the only real resource record in this subdomain
would be that MX record.  At this point, this MX record will point to the
parent domain's mail server (MX record), but that will change when we turn
on the new mail servers.

In any case, we don't need to delegate this subdomain, so it should be
pretty simple, right?  Can I just create an INCLUDE file to add something
like this to the end of the parent domain's zone file:

include file:  spcl.zixvpm.mycompany
====================================
$ORIGIN zixvpm.mycompany.com.
		IN	MX	20	mail.mycompany.com.
====================================


Then add this to the end of the 'db.mycompany.com' zone file:
====================================
...
mail		IN	A		208.123.456.78
$INCLUDE spcl.zixvpm.mycompany
====================================

Is this correct?
Is there any easier way to implement this "DNS mail routing" capability?
Are there any other issues I should consider when doing something like this?

Thanks in advance for any help!
-Bill Bell