Can anyone tell me what's wrong with this domain?

Hi folks.

I run a domain called literacytent.org, which is a small domain
providing a variety of mostly web-based services and hosting.

A while back I set up a subdomain "lists.literacytent.org" for running
mailing lists on. I still get the occasional rejected message to a
subscriber with this sort of message:

Jul 4 12:17:43 shagrat qmail: 1057335463.716148 delivery 42901:
deferral:sender_was_rejected./Remote_host_said:_450_4.4.1_Sender_domain_n
ot_found_in_DNS/

Many variants on that flavor. I must have something wrong in my zone
file, but I can't see what it is. I am NOT a DNS expert, and would love
the advice of someone who is.

This is on a Red Hat 7.3 machine, running BIND 9.2.1 in a chroot jail.
"silicongoblin.com" is my "main" domain, "shagrat.silicongoblin.com" is
the full name of the machine, and "mail.silicongoblin.com" is an alias
for the same machine, for outbound mail. All of that works fine.
"dns-auth[12].crocker.com" are my colo provider's nameservers, set up as
slaves for my virtual hosts so they can change my IP if needed and not
have everbody need to update their DNS records.

The most recent change I added was the explicit MX record for the
lists.literacytent.org subdomain, along with the A record, but some
servers still report the "not in DNS" issue.

Can anybody tell me what is wrong or missing, or what additional
information I'd need to provide? TIA.

- Steve Linberg

From /etc/named.conf:

================================================== ===============
zone "literacytent.org" {
notify yes;
type master;
file "literacytent.org";
};
================================================== ===============

From /var/named/literacytent.org:

================================================== ===============
literacytent.org. IN SOA shagrat.silicongoblin.com.
steve.silicongoblin.com. (
2002052001 ; Serial
8H ; Refresh
2H ; Retry
1W ; Expire
1D ) ; Minimum

TXT "The Literacy Tent"
NS shagrat.silicongoblin.com.

NS dns-auth1.crocker.com.
NS dns-auth2.crocker.com.

MX 10 mail.silicongoblin.com.
lists.literacytent.org. IN MX 10 mail.silicongoblin.com.

localhost IN A 127.0.0.1
literacytent.org. IN A 205.246.6.15
lists.literacytent.org. IN A 205.246.6.15

www.literacytent.org. CNAME literacytent.org.
================================================== ===============