Start / Stop Mail Queue on Exchange 2003

In this case you have to implement security logic, the .Net provides an entire API  System.Security, you can create a permission set that determines rule of messages transactions 

I mean every outgoing mail of a company (mail from a internal network to outside the network)

What do you mean by outgoing mail, do you mean from example a spam?

Thank you very much for your answer!

Probably I should explain a little bit more about the functionality I would like to implement. Basicaly I would like to check every outgoing mail. Therefore I would like to
1. stop the smtp queue
2. wait for messages
3.check these messages (allowed or unallowed and probably remove some of them)
4. Start the smtp queue, so the messages can be send

If I well understand your question you're using a Message queue object then you can do

MessageQueue mqueue = new MessageQueue();
            //This method will make the message queue in a standby state for any message
            IAsyncResult Async = mqueue.BeginReceive();

            //TO do perform you code here

            //This method
            Message Recived = mqueue.EndReceive(Async);
           
            //close the message queue
            mqueue.Close();
It's better to do that asynchormously