i have an Exchange SMTP server but I want to enable this SMTP just for my email addresses (i don't want an open rely mail). How can I block all other recipients? Thanks.
Exchange server only delivers emails of domains it has listed in it's accepted domains.
Add your domain to this list and it should be fine, you can try and remotely connect to your smtp server and send email via another domain, it won't work.
If you want a more detailed description of how to do this you will have to be more specific, like what version of exchange you're running etc.