Setup of DNS Server inside a VirtualBox

Delegating name service

You must ask your domain registrar, Marcaria, to arrange that the nameservice for example.com be delegated to your nameservers. You'll have to give them the Internet-facing publicly-accessible IP-addresses of those nameservers.

In the case of example.com the delegation is carried out by configuring the nameservers for the .com domain. In your case it may be different but your domain registrar should arrange this for you.

Configuring other computers

Most businesses operate split-DNS where the DNS namespace visible to internal users is separate from that available to outside users. This often means that internally you use a separate nameserver for the example.com domain. Either way, for obvious reasons, you specify the nameserver the computer is to use by the nameserver's IP-address, not it's name.

Perhaps the DNS only listen to localhost. Check in the VM :

# netstat -naptu | grep 53

It should show you whether port 53 only listen to localhost (127.0.0.1). If this is the case then you have to check your configuration on allow-query.

Reference on allow-query : http://www.zytrax.com/books/dns/ch7/queries.html