Doubt about how bind9 works

I've being trying to install a mail server using postfix and dovecot-pop3d to be used on a lan (locally).

I order to do that I've installed bind9, created a direct zone with the following content.

$TTL 86400 mydomain.org. IN SOA server1.mydomain.org. admin.mydomain.org. ( 2011082301 10800 3600 604800 38400 ) @ IN NS server1.mydomain.org. @ IN MX 1 server1.mydomain.org. server1.mydomain.org. IN A 192.168.7.10 

When I go to a another pc on the network and try to configure Thunderbird, if I introduce [[email protected]](mailto:[email protected]) it complaints that can not find the settings for the email account. However, if I use [[email protected]](mailto:[email protected]) it works fine but that's not what I want.

If I add mydomain.org. IN A 192.168.7.10 to the direct zone file configuration then I can use [[email protected]](mailto:[email protected]) to get the account settings from the server.

My problem is understanding how this works. According to what I have researched

@ IN NS server1.mydomain.org. server1.mydomain.org. IN A 192.168.7.10 

this two lines should already inform that mydomain.org can be found in 192.168.7.10. I do not understand why I still need to add mydomain.org. IN A 192.168.7.10 in order to work properly.

I would appreciate any insight