origin substitution in zone file

[wolf2k5]

Hi,

I am running Bind 9.

My domain zone file contains the following records for mail, I
replaced the real IP address with 127.0.0.1 and the real domain with
mydomain.com for privacy:

@    IN    MX    10 inbound.mydomain.com.mailprovider.com.

I'd like to use a sobstitution of the zone origin for the MX record
instead of hardcoded mydomain.com value in it (since I use the same
zone file for multiple domains):
@    IN    MX    10 inbound. at mailprovider.com.

I tried that, but that doesn't work as expected, if I reload the zone
file and query the nameserver for the MX record for the domain, I get
the following result:
mydomina.com.    14400    IN    MX    10 inbound.\@emailmx.com.coresecure.n=
et.

It looks BIND only runs the substitution on the left side of the record.

Is there any way to force it to the right side too?

Is there any other solution to use the same zone file for multiple domains?

Thanks.