e.g: a user can only send and receive to one email address (@yahoo.com)regards

If you have Exchange 2007/2010 tiy you can do this with transport rules.

now we using exchange 2003? it is possible?thank's

I do not know. I'm afraid I don't have any 2003 left in my org to test with.

No. Exchange 2003 transport does not allow you configuring any settings like that. You can check for some third party solution at gateway level if you want to do that.Milind Naphade | MCTS:M (Exchange 2007 and 2010) | http://www.msexchangegeek.com

You can try setting up a custom send connector for just that address, and then setting your send connector permissions so that the user can only use that send connector. If that works, it will lock them down to just sending to that address (at least externally). On the inbound side, you may be able to control it from the receive restrctions on the mailbox.