Problem with SOA record and syslog error

I need an expert to help solve this problem!

We are recieving this error message in one of our log files:

Jan 8 14:28:27 ns named[411]: sysquery: findns error (NXDOMAIN) on ns.24hourmall.com?
Jan 8 14:28:27 ns named[411]: sysquery: findns error (NXDOMAIN) on ns2.24hourmall.com?

This happens every second (repeats) !
I think the problem lies in the zone files for our domain. Here is a copy of it:

$TTL 3600
@ IN SOA ns.24hourmall.com. postmaster.ns.24hourmall.com. 2002012101 10800 3600 604800 3333
24hourmall.com. IN MX 10 mail.24hourmall.com.
24hourmall.com. IN NS ns.24hourmall.com.
24hourmall.com. IN NS ns2.24hourmall.com.
@ IN A 64.239.37.16
ftp.24hourmall.com. IN A 64.239.37.16
mail.24hourmall.com. IN A 64.239.37.16
URL

My first question is why is there an @ symbol on the second line instead of 24hourmall.com. ? Why is our email address postmaster.ns.24hourmall.com. , rather than postmaster.24hourmall.com. ? What are the other problems with these records that would make it generate the error? Thanks for your help!