Hello,
In order to have greater control over the emails sent out the company I imagined the following scenario.
Scenario

Describing in words what I need.

User sends an email to user@hotmail.com for example, using the smtp of my isp (is the internet).
Before the email is sent, the firewall checks if the recipient is blocked transparent, if you are locked out rejects the email.

I searched several forums and mailing lists, but did not find scenarios similar to mine.
Where found situations in which the mail servers were internally in the company, which is not my case.

I have no control over the smtp of my isp.

I do not need a howto, just a direction how to proceed.

Thanks for your help!

Well there are transparent SMTP servers, e.g. pop3scan as a very basic example, why would you bother? They directly the ISP SMTP server? Why? If you can configure that, why not configure them to use a local one instead??

Becouse my necessity is to block some destinations. Ex.: i would like to block all e-mails sended to bob12345@yahoo.com.

All email sent to certain emails I would like to block.

To ensure that information does not reach unauthorized persons.