i just want to configure a subdomain, but not to delegate the whole
subdomain zone to another name server.

For example,
main domain: domain.net
subdomain: sub.domain.net

i tried with this:

(zone file /var/named/master/domain.net)
--
@ IN SOA ns1.domain.net. hostmaster.domain.net. (2005031103 3600
600 86400 3600)
IN NS ns1.domain.net.
IN NS ns2.domain.net.
IN MX 10 mail
ns1 IN A 192.168.0.3
ns2 IN A 192.168.0.4
$ORIGIN sub.domain.net.
IN MX 10 smtp1
smtp1 IN A 192.168.0.25
--

But running the 'host' command:
# host -t mx sub.domain.net
Host sub.domain.net not found: 3(NXDOMAIN)

So, changin the configuration of the main domain zone file by this:
--
@ IN SOA ns1.domain.net. hostmaster.domain.net. (2005031103 3600
600 86400 3600)
IN NS ns1.domain.net.
IN NS ns2.domain.net.
IN MX 10 mail
ns1 IN A 192.168.0.3
ns2 IN A 192.168.0.4
sub.domain.net. IN MX 15 smtp1.sub.domain.net.
smtp1.sub.domain.net. IN A 192.168.0.25
--

the host command result is:
# host -t mx sub.domain.net
sub.domain.net mail is handled by 15 smtp1.sub.domain.net.

I dont understand why, i've only replaced $ORIGIN by the fqdn
sub.domain.net.
What was wrong in the first configuration?

Tkanks..